Catalyzes ATP-dependent phosphorylation of D-ribulose at C-5 to form D-ribulose 5-phosphate. Postulated to function in a metabolite repair mechanism by preventing toxic accumulation of free D-ribulose formed by non-specific phosphatase activities. Alternatively, may play a role in regulating D-ribulose 5-phosphate recycling in the pentose phosphate pathway. Can phosphorylate ribitol with low efficiency. ATP + D-ribulose = ADP + D-ribulose 5-phosphate + H(+) Carbohydrate metabolism; pentose and glucuronate interconversion. Belongs to the FGGY kinase family.