Senna siamea is a medium-size, evergreen tree growing up to 18 m tall, with a straight trunk of up to 30 cm in diameter; bole short, crown usually dense and rounded at first, later becoming irregular and spreading with drooping branches. Bark grey or light brown, smooth but becoming slightly fissured with age. The root system consists of a few thick roots, growing to considerable depth, and a dense mat of rootlets in the top 10-20 cm of soil, which may reach a distance of 7 m from the stem in 1 year and eventually a distance up to 15 m. Leaves alternate, pinnately compound, 23-33 cm long, with slender, green-reddish, tinged axis; leaflets 6-12 pairs on short stalks of 3 mm, oblong, 3-7 cm long, 12-20 mm wide, rounded at both ends, with tiny bristle tip. Flower clusters are upright at ends of twigs, large branched, 20-30 cm long, 13 cm broad, with many bright yellow flowers 3 cm across, pentamerous; sepals imbricate, obtuse at the apex; petals subequal to heteromorphic, yellow; stamens 10, accrescent toward the abaxial side of the flower; filaments straight and not more than twice as long as the anthers; ovary superior, linear and curved. Pods numerous, long, narrow, 5-25 cm long, 12-20 mm broad, flat, dark brown, strap shaped, stipitate, terete to compressed, dehiscent, with septae between the numerous seeds; seeds are bean shaped, shiny, dark brown, 8 mm long, with distinct areole.