This key adapted from the Flora of North America.  Additional entries and information have been added from other sources.
1   Flower pedicellate; bract blades usually not mottled. (2)  
+   Flower sessile, bract blades usually mottled (except in T. pusillum). (19)  
         
2 (8) Stigmas ± connate at least basally, or closely grouped and uniform, of uniform thickness throughout. (3)  
+   Stigmas distinct, often basally thickened or subulate. (11)  
         
3 (2) Scapes usually 0.4–2 dm at anthesis. (4)  
+   Scapes usually 2.0–4.5 dm at anthesis. (7)  
         
4 (3) Bract blade bluish green, apex round-obtuse; scapes 6-gonal in cross section. 6 Trillium nivale
+   Bract blade green, often with maroon undertones early, apex acuminate, blunt-acute, or rounded; scapes round in cross section. (5)  
         
5 (4) Pedicel recurved below bracts at anthesis, or rarely leaning to ± horizontal distal to bracts; petals strongly arcuate-recurved from base, white, pink, or rose. 1 Trillium catesbaei
+   Pedicel ± erect at anthesis; petals erect-spreading to widely spreading, not recurved or recurved only in distal 1/2, white or white with madder purple flecks. (6)  
         
6 (5) Bracts strongly petiolate, blade apex long-acuminate, weakly to strongly cordate basally, glossy, bluish green, sometimes with lighter silvery green pattern on or near main veins; sepals shorter and narrower than petals, apex rounded-apiculate; petals erect-spreading, ovate-cordate to ± orbicular, widest at or beyond middle, round-tapered distal to middle to apiculate-acuminate tip, margins mostly without undulations, white, usually sparsely to very heavily flecked with madder-purple, color not changing with age; Siskiyou Mountains of n California and Oregon. 10

Pseudotrillium rivale

(Trillium rivale)
+   Bracts very short-petiolate, blade apex obtuse, not glossy, green with maroon undertones; sepals about as large and prominent as petals, apex round to blunt; petals spreading-ascending, weakly recurved in distal 1/2, oblong to narrowly lanceolate, widest in proximal 1/3, margins strongly undulate, white, aging to deep rose abaxially; e U.S. 9 Trillium pusillum  
A   Flowering rhizomes slender, to 1/4" thick & freely branching (B)  
A   Flowering rhizomes stout, to 1/2" thick (E)  
B   Flower sessile or subsessile; pedicel, if present, 0.1–0.3 cm 9h T. p. v. virginianum
B   Pedicel 1/4" or longer (C)  
C   Leaves with stomate pores above (D)  
C   Leaves medium green, without stomates above. 9f T. p. v. pusillum
D   Leaves ascending, grey green and appearing dusty, 3/4" wide 9g T. p. v. texanum
D   Leaves not ascending, blue-green 1/2" wide 9b T. p. v. georgianum
E   Rhizomes unbranched and with crowded nodes 9e T. p. v. ozarkanum
E   Rhizomes freely branching & with prominent internodes (F)  
F   Plants robust, to 8", petals to 1.5" 9c T. p. v. kentuckianum
F   Plants small, to 6" petals to 1.0" long 9a T. p. v. alabamicum
      9d T. p. v. monticulum
         
7 (3) Stigmas connate into style 2–6 mm; bract blades elliptic-ovate, ovate-lanceolate to lanceolate, usually 1.5–8 cm wide. (8)  
+   Stigmas barely connate basally, rarely completely distinct; bract blade ovate to rhombic, usually 8–20 cm wide. (9)  
         
8   Pedicel erect or only slightly leaning; petals erect to widely spreading, linear to narrowly elliptic, white at anthesis, darkening with age to light to medium deep pink, with inverted V-shaped, white base; bracts horizontal to drooping distally, blade lanceolate to lanceolate-ovate; stamens straight. 8 Trillium persistens
+   Pedicel leaning to almost horizontal or clearly recurved and declined; petals arcuate-recurved, oblong-lanceolate, white, pink, or rose, darkening to pink or nonfading with age; bracts and their margins often slightly raised, exposing flowers at anthesis, blade elliptic-ovate; stamens recurved. 1 Trillium catesbaei  
         
9 (7) Petals with distinct, dark red, inverted V-shaped mark basally; bracts strongly petiolate; ovary 3-lobed or -angled 14 Trillium undulatum
+   Petals uniformly white or pink, or with faint pink blush, no distinct, V-shaped marking; bracts subsessile, sessile, or short-petiolate; ovary 6-lobed or -angled. (10)  
         
10 (9) Petals erect basally, flaring and recurving above middle, margins undulate-wavy, overlapping basally, rolled, forming tube that conceals ovary when viewed from above, obovate to lanceolate, rarely suborbicular, 4–7.5 × 2–4 cm, white, aging to pink-purple; e North America. 5 Trillium grandiflorum  
    Overall plant size 1/2 or less that of T. grandiflorum, approx. 15cm H. 5a T. g. v. parviflorum
    Flower opens pink, ages deeper pink 5b T. g. v. roseum
+   Petals erect-ascending, widely spreading in distal 1/2, distal margins usually flat to weakly undulate to undulate, scarcely touching basally to overlapping and forming tube, not concealing ovary when viewed from above, linear to obovate, 1.5–7 × 1–4 cm, white, often with pink or blush markings, aging to deep rosy pink, purple, or dark red; w North America. 7 Trillium ovatum  
    Overall plant proportions are 1/2 or less that of  T. ovatum 7.5-15cm H.  Flower opens pink, turns white, ages back to pink 7a T. o. v. hibbersonii
    Bracts green with dark brownish green mottling or spotting 7b T. o. v. maculosum
    Bracts distinctly short-petiolate 0.3-1.0mm green. Pedicel ± leaning to almost horizontal. Flower nodding; petals linear to linear-lanceolate, 0.5–2.4 × 0.2–0.6 cm; 7c T. o. v. oettingeri
         
11 (2) Stigmas short, 2–4 mm, uniformly thick, recurved and appearing ± lobed adaxially, at least in dried specimens; bracts strongly petiolate, blade ovate-lanceolate, mostly less than 6 cm wide, weakly to strongly cordate basally. 10 Psuedotrillium rivale (Trillium rivale)
+   Stigmas of various lengths, basally thickened, gradually tapered, erect distally, reflexed, or coiled, not lobed adaxially; bracts sessile to subsessile, blade elliptic, obovate, or rhombic, mostly more than 6 cm wide, gradually tapered, rounded or weakly cuneate basally. (12)  
         
12 (11) Anthers ± equaling filaments, pale lavender-pink or -gray; petals oblong-lanceolate, recurved-reflexed from just above base, extending behind plane of sepal bases for more than 1/2 their length, white or rarely pale pink, thin-textured, adaxial veins inconspicuous. 2 Trillium cernuum  
+   Anthers longer or shorter than, rarely equaling filaments, white, creamy white, yellow, purple, or maroon; petals ovate to obovate, spreading, flat, or, if recurved, in distal 1/2 only, red, maroon purple, rose, cream, or white, more rarely pale yellow, rarely 2-colored, heavy- or fleshy-textured, adaxial veins conspicuous, appearing engraved. (13)  
         
13 (12) Ovary greenish white or white, occasionally with reddish purple stains at apex. (14)  
+   Ovary dark purple or maroon. (15)  
         
14 (13) Pedicel strongly recurved beneath bracts; anthers thin, dark purple; petals ovate-elliptic, white, rarely 2-colored rose and white, apex acuminate; stigmas shorter than ovary; rhizome tapered to point distally. 11 Trillium rugelii
+   Pedicel erect, declined, or recurved; anthers thick, creamy white or yellow; petals ovate-lanceolate to broadly ovate, creamy white, not 2-colored, apex acute; stigmas ± equaling ovary; rhizome praemorse. 4 Trillium flexipes
         
15 (13) Pedicel recurved or declined; flower held proximal to or below bracts. (16)  
+   Pedicel erect, or leaning from base, not recurved or declined; flower erect or dangling but not distinctly below bracts. (17)  
         
16 (15) Pedicel declined, ± straight; stamens 1.5–2 times taller than small, ± globose to conical-pyramidal ovary; anthers maroon to grayish purple; petals 3–6.5 × 4–6 cm, usually maroon to brownish red, ovate-suborbicular, spreading to weakly recurved, longer than sepals. 15 Trillium vaseyi  
+   Pedicel recurved beneath bracts; stamens ± equaling or not more than 1/3 longer than prominent, flask-shaped ovary; anthers dark purple; petals 2.5–5 × 0.8–3.5 cm, usually white or 2-colored, distally white, ovate to broadly ovate, recurved, ± equaling sepals 11 Trillium rugelii  
         
17 (15) Sepals open, usually flat; petals red, purple carried in ± same plane as sepals, or ascending slightly, spreading, not recurved, lanceolate to ovate, to 2+ times width of sepals; sepals flat to somewhat sulcate apically; flower odor fetid, of wet dog. 3 Trillium erectum  
+   Petals white 3a Trillium erectum v. album
+   Sepals gaping, strongly 3-dimensional; petals erect-spreading or recurved in distal 1/2, ovate to ovate-orbicular, wider than sepals; sepal apex sulcate; flower odor musty or fruity. (18)  
         
18 (17) Petals creamy white, 1.5 times sepal length, 4–7 × 1.5–4 cm, not recurved to weakly so at tip; pedicel not strongly reflexed, flowers facing ± upward, odor sweet, apple like; sepal apex mildly sulcate. 12 Trillium simile  
+   Petals dark reddish maroon to purplish, less than 1.5 times sepal length, 1.8–5 × 1–3 cm, often recurved in distal 1/2; pedicel reflexed at tip; flowers facing outward at ± right angle to pedicel, odor fungus like; sepal apex strongly sulcate. 13 Trillium sulcatum  
         
19 (1) Stigmas connate basally into short style, threadlike, tips widely spreading; petals spreading from base with distal 1/2 flat or weakly ascending, exposing stamens and ovary, of short duration, margins undulate 9 Trillium pusillum
+   Stigmas distinct, erect, subulate with tips divergent-recurved, or linear and usually slightly thickened basally with tips recoiled; petals mostly erect from base, ± connivent, often at least partially concealing stamens and ovary, or erect-spreading, tips incurving slightly, long-lasting, sometimes spiral-twisted, margins entire. (20)  
         
20 (19) Scapes decumbent or semi decumbent, or subterranean; bracts resting near or on ground, or with petioles arising from scape apex at or near ground surface. (21)  
+   Scapes erect, straight; bracts held well above ground, or bract tips touching ground only in early anthesis. (23)  
         
21 (20) Scapes ± erect, largely subterranean; bracts long-petiolate, blade not mottled; individual bracts Plantago-like; w United States. 31 Trillium petiolatum
+   Scapes decumbent or semi decumbent, S-shaped, aerial; bracts sessile or only short-petiolate, blade heavily mottled; se United States. (22)  
         
22 (21) Anther dehiscence extrorse; scapes distinctly puberulent especially just below bracts; petals linear-lanceolate or oblanceolate, 4–8 cm or more; filaments 2–5 mm. 21 Trillium decumbens  
+   Anther dehiscence introrse; scapes glabrous; petals narrowly elliptic-oblanceolate, 2.5–5.5 cm; filaments 1–2 mm. 33 Trillium reliquum  
         
23 (20) Sepals strongly reflexed at bases, often below bracts and parallel to scape, or adpressed to bracts; anther connectives strongly curved inward; rhizomes horizontal, elongated, slender, very brittle, white. (24)  
+   Sepals not strongly reflexed, spreading or somewhat erect; anther connectives straight; rhizomes horizontal or erect, thick or somewhat compressed-thickened and superficially bulblike, not brittle, brownish. (25)  
         
24 (23) Petals lanceolate-ovate, base attenuate to weakly clawed, ca. 2 times longer than wide; bracts petiolate. 32 Trillium recurvatum
+   Petals linear to narrowly spatulate, clawed, at least 4 times longer than wide; bracts sessile. 26 Trillium lancifolium
         
25 (23) Petals white, rarely tinged pale pink or purplish near bases. (26)  
+   Petals maroon, reddish brown, bronze, yellow, or green. (28)  
         
26 (25) Anther dehiscence introrse; ovary purple. 18 Trillium chloropetalum v. album
+   Anther dehiscence latrorse; ovary green or greenish white, rarely purple at base. (27)  
         
27 (26) Scapes usually 2.2–5.8 dm; bracts sessile, petals oblanceolate to obovate, 4.8–8 cm, white to creamy white, rarely very pale pink or pinkish basally, odor rose like. 16 Trillium albidum
+   Scapes usually 1.7–3 dm; bracts sub-sessile, petals linear to linear-lanceolate, 2.2–4.5 cm, white, rarely purplish basally, odor spicy. 30 Trillium parviflorum
         
28 (25) Petals yellow, pale yellow, greenish, or bicolored green with purplish base. (29)  
+   Petals usually maroon, reddish brown, pink, greenish maroon, purplish bronze, white, or brown throughout (except in pigment-free mutant individuals with petals pale green or yellowish green); occasionally yellow in T. cuneatum, T. chloropetalum, T. gracile, T. maculatum, and T. foetidissimum, occasionally 2-colored in T. ludovicianum. (33)  
         
29 (28) Petal bases not clawed. (30)  
+   Petal bases weakly to strongly clawed. (31)  
         
30 (29) Petals oblanceolate to lanceolate, apex long-acuminate; stamens 11–18 mm; anther connective extending 0.5 mm or less beyond sacs; stamens slightly longer than or ± equaling pistil; flower odor strongly of lemon; se United States. 28 Trillium luteum 
+   Petals oblanceolate to obovate, apex variably acute to almost truncate; stamens 17–26 mm; anther connective extending 1–1.5 mm beyond sacs; stamens ca. 2 times as long as pistil; flower odor spicy, of rose; California. 18 Trillium chloropetalum v. chloropetalum
         
31 (29) Petals widely spatulate, pale sulfur-yellow fading to ± cream, with broadly cuneate basal claw, claw green, sometimes obscurely marked with purple; apex apiculate, nipple like in at least 1 petal. 22 Trillium discolor  
+   Petals narrowly spatulate to linear-spatulate, green, bicolored, green with purplish base, rarely all purple, obscurely to clearly narrow-clawed basally, claw purple or green; apex round-acute to obtuse, lacking nipple. (32)  
         
32 (31) Bract (under magnification) with numerous stomates across entire adaxial surface; pistil 2/3 stamens; anther dehiscence introrse-latrorse; petals green or yellowish green distal to weak purplish claw (very rarely purplish green throughout); flower odor of rotten fruit. 37 Trillium viride
+   Bract adaxially without stomates or with stomates near apex only; pistil 1/2 stamens; anther dehiscence latrorse; petals green or yellowish green distal to distinct purplish claw (rarely, petal purple throughout); flower odor spicy or musty. 38 Trillium viridescens 
         
33 (28) Anther dehiscence extrorse; stamens not obscured by petals; petals horizontal, with 1–2 spiral twists. 35 Trillium stamineum
+   Anther dehiscence introrse or latrorse; stamens at least partially obscured by petals; petals erect to divergent and slightly spreading with tips ± incurved, not spirally twisted. (34)  
         
34 (33) Anther dehiscence latrorse, occasionally introrse basally with age. (35)  
+   Anther dehiscence introrse. (38)  
         
35 (34) Anther connectives extended 1–2 mm beyond sacs; margins of distal 1/3 of bract blade straight to apex. (36)  
+   Anther connectives extended scarcely if at all to 0.5 mm beyond sacs; margins of distal 1/3 of bract blade convex-curved to apex. (37)  
         
36 (35) Scape 1.5 times longer than bracts; bract tips at anthesis often touching ground; petals 3–4 times longer than wide. 36 Trillium underwoodii
+   Scape 2.5–3 times longer than bracts; bract tips at anthesis held well away from ground; petals ca. 2 times longer than wide. 20 Trillium decipiens
         
37 (35) Petals 0.9–2.7 cm wide, oblanceolate to elliptic-obovate, narrowed to cuneate but not clawed base, erect. 19 Trillium cuneatum
+   Petals 0.4–0.8 cm wide, linear-oblanceolate, weakly clawed basally, erect-spreading. 27 Trillium ludovicianum
         
38 (34) Stamens ca. 2 times as long as pistil. (39)  
+   Stamens equaling, less than, or very slightly longer than pistil. (40)  
         
39 (38) Petals oblanceolate to obovate, 6.5–10 × 1.5–2.5 cm; anther connective extended 1–1.5 mm beyond sacs; California. 18 Trillium chloropetalum v. giganteum
+   Petals linear-elliptic to oblanceolate, 2–4 × 0.3–0.8 cm; anther connective extended 0.1–1 mm beyond sacs; e Texas and w Louisiana. 24 Trillium gracile
         
40 (38) Anther connective not extended beyond sac, or, if extended, rarely to 0.5–1 mm. (41)  
+   Anther connective extended 1 mm or more beyond sac. (42)  
         
41 (40) Scape 1.4–4 dm; bracts 7–15 × 4.6–6.7 cm; petals narrowly oblanceolate-spatulate to linear-spatulate, spreading-erect, becoming connivent-erect with age, 4–7 × 0.7–1.7 cm, clear dark red-maroon, reddish purple, or rarely yellow above purplish base or clear sulfur yellow, distal margins upraised from about widest point to near petal base; stigmas 2–4 mm; se United States. 29 Trillium maculatum  
+   Scape 2.5–5.5 dm; bracts 11–18 × 12–17 cm; petals oblanceolate, spreading-erect, 5.5–11 × 2–3.5 cm, glossy dark maroon-red or purple, quilling with age, margins ± flat, widest at or below middle, cuneate basally; stigmas 6–8 mm; n California and extreme sw Oregon. 25 Trillium kurabayashii  
         
42 (40) Anther connective extended 2–5+ mm beyond sac. 34  Trillium sessile
+   Anther connective extended ca. 1–1.5 mm beyond sac. (43)  
         
43 (42) Scapes 2.5–6 dm; petals linear, erect, 5–10 cm, 8–10 times longer than wide, glossy, dark purple to red-purple, inrolling/scrolling with age; bract blades 10–22 × 8.7–15 cm, obscurely and sparsely or not at all mottled or spotted darker greenish brown, subsessile; c, s California. 17 Trillium angustipetalum  
+   Scapes 0.8–2.8 dm; petals narrowly elliptic to linear-lanceolate, weakly incurved, 2–5 × 0.3–0.5 cm, not glossy, pinkish purple, light to dark reddish or brownish purple, rarely yellow, not inrolling/scrolling with age; bract blades 6.7–12 × 3.8–6 cm, strongly mottled, sessile; e Louisiana, sw Mississippi. 23 Trillium foetidissimum