Trillium underwoodii, commonly known as the longbract wakerobin, is a species of flowering plant in the family Melanthiaceae. It is native to the southeastern United States, where it occurs in Alabama, Florida, and Georgia. It is a perennial herb that spreads by underground rhizomes.
Trillium underwoodii is a perennial herb up to 20 cm tall, spreading by means of underground rhizomes. The leaves are mottled with light, medium, and dark green blotches. Flowers are foul-smelling or musk-scented, usually deep maroon or purplish red but occasionally yellow. Small described the sepals as lanceolate and the berry as ovoid.
The species was first described by John K. Small in 1897. In his original treatment, Small distinguished T. underwoodii from Trillium sessile, noting differences in overall size and habit as well as in floral characters such as the stamens and styles. A 2019 molecular phylogenetic study found little genetic distinction between Trillium underwoodii and Trillium decipiens, suggesting that their status as separate species may require re-evaluation.
According to a 2019 assessment by the Trillium Working Group, Trillium underwoodii is considered a species of least concern globally, with a NatureServe conservation status of G4 ("apparently secure").
The species occurs across southern Alabama, southwestern Georgia, and northern Florida. Although population data are limited, some subpopulations are reported to be large. Potential threats include habitat degradation from logging and urbanization, herbivory by overabundant white-tailed deer, disturbance from feral pigs, and competition from invasive plant species such as Japanese stiltgrass, bush honeysuckle, English ivy, Chinese privet, and trifoliate orange. However, no widespread population declines have been documented.