Cyclin is a protein involved in the regulation of the cell cycle. It forms a complex with the cyclin-dependent kinase (Cdk), which activates the latter's protein kinase function. Cyclins are named because their concentration varies in a cyclical fashion during the cell cycle.

There are several different cyclins which are active in different parts of the cell cycle and which cause the Cdk to phosphorylate different substrates.

One major cyclin is cyclin B, a mitotic cyclin. The amount of cyclin B, and the activity of the cyclin B-Cdk complex, rises through the cell cycle until mitosis. It falls abruptly during mitosis due to the degradation of cyclin B. The complex of Cdk and cyclin B is called maturation promoting factor (MPF). Other cyclins include cyclin E (binds to G₁ phase Cdk), which is required for the transition from G₁ to S phase and cyclin A (binds to S phase Cdk) and is required for the cell to progress through the S phase.

Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of cyclin and cyclin-dependent kinase, central molecules in the regulation of the cell cycle.