P-selectin is a protein that in humans is encoded by the SELP gene. P-selectin functions as a cell adhesion molecule (CAM) on the surfaces of activated endothelial cells, which line the inner surface of blood vessels, and activated platelets. In unactivated endothelial cells, it is stored in granules called Weibel-Palade bodies. In unactivated platelets P-selectin is stored in α-granules.

P-selectin plays an essential role in the initial recruitment of leukocytes (white blood cells) to the site of injury during inflammation. When endothelial cells are activated by molecules such as histamine or thrombin during inflammation, P-selectin moves from an internal cell location to the endothelial cell surface.

P-selectin has a functional role in metastasis of tumor similar to E-selectin.[16] P-selectin is expressed on the surface of both stimulated endothelial cell and activated platelet and helps cancer cells invade into bloodstream for metastasis and provided locally with multiple growth factors respectively.[17] Moreover, it has been known that platelet facilitates tumor metastasis by forming complexes with tumor cells and leukocytes in the vasculature thus preventing recognition by macrophage, this is thought to contribute to the seeding of tumor microemboli to distant organs.[18] In vivo mice experiment showed that reduction in circulating platelets could reduce cancer metastasis.