Ribulose is a ketopentose — a monosaccharide containing five carbon atoms, and including a ketone functional group. It has chemical formula ₅₁₀₅. Two diastereomers are possible, D-ribulose (D-erythro-pentulose) and L-ribulose (L-erythro-pentulose). D-ribulose is an intermediate in the fungal pathway for D-arabitol production.

As the 1,5-bisphosphate, it combines with CO2 at the start of the photosynthetic process in green plants (carbon dioxide trap); d-ribulose is the epimer of d-xylulose.