Significance Underground barriers caused by drought or heterogeneous soil components exert mechanical resistance to plants, and plant roots must determine whether to avoid or penetrate these barriers. Yet, very little is known about how roots sense mechanical forces at the molecular level. Using genetics, electrophysiology, and in vivo imaging, we found that Arabidopsis PIEZO1, an ortholog of mammalian Piezo, is required for root penetration and that pzo1 mutants have impaired primary root growth into harder media. These results highlight a conserved function of PIEZOs from plants to mammals.