PIEZO1 is a cation channel that is activated by mechanical forces such as fluid shear stress or membrane stretch. loss-of-function mutations in patients are associated with congenital lymphedema with pleural effusion. However, the mechanistic link between PIEZO1 function and the development or function of the lymphatic system is currently unknown. Here, we analyzed two mouse lines lacking PIEZO1 in endothelial cells (via or ) and found that they exhibited pleural effusion and died postnatally. Strikingly, the number of lymphatic valves was dramatically reduced in these mice. Lymphatic valves are essential for ensuring proper circulation of lymph. Mechanical forces have been implicated in the development of lymphatic vasculature and valve formation, but the identity of mechanosensors involved is unknown. Expression of FOXC2 and NFATc1, transcription factors known to be required for lymphatic valve development, appeared normal in mice. However, the process of protrusion in the valve leaflets, which is associated with collective cell migration, actin polymerization, and remodeling of cell-cell junctions, was impaired in mice. Consistent with these genetic findings, activation of PIEZO1 by Yoda1 in cultured lymphatic endothelial cells induced active remodeling of actomyosin and VE-cadherin cell-cell adhesion sites. Our analysis provides evidence that mechanically activated ion channel PIEZO1 is a key regulator of lymphatic valve formation.