Sannantha crassa is a shrub that typically grows to a height of up to and has grey, scaly bark. Its leaves are elliptic to lance-shaped, long and wide on a petiole long. The flowers are up to in diameter and arranged in leaf axils in groups of 7 to 9 on a peduncle long. Each flower is on a pedicel long with 2 bracts at the base, but that fall off as the flowers develop. The floral tube is long, the sepal lobes long and thin. The petals are white, long and wide and there are 8 to 11stamens. Flowering has been observed in January and February and the fruit is a bell-shaped to hemispherical capsule in diameter.