The specific epithet sessile means "attached without a distinct stalk",[4] an apparent reference to its stalkless flower.

For instance, T. sessile and T. recurvatum are common to at least nine U.S. states (Alabama, Arkansas, Illinois, Indiana, Kentucky, Michigan, Missouri, Ohio, Tennessee).

[11] The latter species has distinctive leaves with petiole-like leaf bases and strongly recurved sepals, but T. sessile has neither of these characteristics.

Its elongated, erect stigmas are usually more than 1.5 times the length of the relatively short, nearly spherical, sharply angled ovary.

Relative sizes and proportions are often useful, however: In locales where two or more sessile-flowered Trillium species come in contact, T. sessile usually flowers earlier.

californicum has been used to refer to a variety of T. chloropetalum, a sessile-flowered Trillium species confined to western North America (where T. sessile in the modern sense does not range).

[19][20] As of January 2022[update], Kew's Plants of the World Online accepts no infraspecific names for Trillium sessile.

[3] Some authorities recognize the name Trillium sessile f. viridiflorum Beyer,[21] a form with yellow-green petals that occurs spontaneously throughout the range of the species.

The latter is distinguished from T. sessile by its larger size, the mottling of its leaves, shorter filaments, the color of its petals, and the character of its stigmas.

[22] Trillium sessile is endemic to the eastern half of the United States, ranging from Maryland in the east to the Ozarks in the heart of the Mississippi Basin, and from the southern Great Lakes region to northern Alabama.

The species occurs in more U.S. states than any other sessile-flowered trillium:[3][6] It is most abundant in Ohio, Indiana, northern Kentucky, and Missouri, but it is strangely absent from central Illinois.

), Hepatica spp., Virginia bluebells (Mertensia virginica), mayapple (Podophyllum peltatum), woods-poppy (Stylophorum diphyllum), and violets (Viola spp.).

It can withstand substantial disturbance and habitat degradation, persisting under light pasturing, along fence rows, and in brushy areas after lumbering.

[29] The above ground parts of the plant die back by mid-summer but may persist longer in areas that do not completely dry out.

[32] It is sometimes cited as having been used as a poultice for boils and as a panacea-like decoction, but this claim is doubtful since it is attributed to Native American tribes (the Yuki and the Wailaki) of California, where this plant is not known to occur.