Migrating cells must adapt to the mechanochemical properties of the microenvironment. Here, we demonstrate that intracellular [Ca2+] in melanoma cells rises in response to 2D confinement. Upon chelating Ca2+ or inhibiting Piezo1, cells were less likely to phenotypically transition from mesenchymal to fast amoeboid migration. In VCAM-1 coated microchannels, melanoma cells adopt a hybrid mode of migration, which has hallmarks of mesenchymal and amoeboid migration. In the absence of Piezo1 activity, cells predominantly adopted a mesenchymal mode of migration. In undulating channels, however, melanoma cells are almost entirely amoeboid. In agreement with a curvature induced increase in plasma membrane tension, this effect could be blocked by inhibiting Piezo1 or chelating Ca2+. In addition to ROCK2, we found that activation of inverted formin-2 (INF2) downstream of Piezo1/Ca2+ to be required for amoeboid migration. Thus, the activation of INF2 by Piezo1/Ca2+ constitutes an essential mechanism by which cells respond to confining environments.