Trillium grandiflorum is a perennial that grows from a short rhizome and produces a single, showy white flower atop a whorl of three leaves. These leaves are often called bracts as the "stem" is then considered a peduncle (the rhizome is the stem proper, aboveground shoots of a rhizome are branches or peduncles); the distinction between bracts (found on pedicels or peduncles) and leaves (borne on stems). A single rootstock will often form clonal colonies, which can become very large and dense.
Detail of a leafy bract
showing engraved venation
The erect, odorless flowers are large, especially compared to other species of Trillium, with 4 to 7 cm (1.5 to 3 in) long petals, depending on age and vigor. The petals are shaped much like the leaves and curve outward. They have a visible venation, though this is nowhere near as marked as on the leaves. Their overlapping bases and curve give the flower a distinctive funnel shape. Between the veined petals, three acuminate (ending with a long point) sepals are visible; they are usually a paler shade of green than the leaves, and are sometimes streaked with maroon. Flowers are perched on a pedicel (i.e., flower stalk) raising them above the leaf whorl, and grow pinker as they age.
Flowers have six stamens in two whorls of three, which persist after fruiting. The styles are white and very short compared to the 9–27 mm (0.35–1.06 in) anthers, which are pale yellow but becomes a brighter shade when liberating pollen due to the latter's color. The ovary is six-sided with 3 greenish-white stigmas that are at first weakly attached, but fuse higher up. The fruit is a green, mealy and moist orb, and is vaguely six-sided like the ovary.