Ribulose is a ketopentose — a monosaccharide containing five carbon atoms, and including a ketone functional group. It has chemical formula C₅H₁₀O₅. Two enantiomers are possible, d-ribulose (d-erythro-pentulose) and l-ribulose (l-erythro-pentulose). d-Ribulose is the diastereomer of d-xylulose.

Ribulose sugars are composed in the pentose phosphate pathway from arabinose. They are important in the formation of many bioactive substances. For example, d-ribulose is an intermediate in the fungal pathway for d-arabitol production. Also, as the 1,5-bisphosphate, d-ribulose combines with carbon dioxide at the start of the photosynthesis process in green plants (carbon dioxide trap).

Ribulose has the same stereochemistry at carbons 3 and 4 as the five-carbon aldoses ribose and arabinose.