A keto-pentose sugar (C5H11O5) that is involved in carbon dioxide fixation in photosynthesis.

Ribulose is a ketopentose — a monosaccharide containing five carbon atoms, and including a ketone functional group. It has chemical formula 5105. Two enantiomers are possible, D-ribulose (D-erythro-pentulose) and L-ribulose (L-erythro-pentulose).