Cancer cells trigger platelet aggregation. Reciprocally, platelet aggregation promotes tumor growth and metastasis. Within the tumor microenvironment, platelet regulates tumor growth via several mechanisms involving stimulation of angiogenesis, inflammation, coagulation, and stabilizing of vessel wall. In circulation, cancer cells coated by platelets can travel to metastatic site protected from intravascular shear forces and from immune surveillance. Finally, platelets facilitate adhesion of tumor cells and formation of the metastatic niche. Platelet-derived microparticles contain growth factors contributing to tumor angiogenesis. On the other hand, platelets can selectively release anti-angiogenic factors in a process connected to tumor dormancy. Therapeutical inhibition of platelet aggregation prevents tumor development in certain tumor types and may contribute to better cancer outcome.