Significance The biconcave disk shape and deformability of the mammalian RBC are vital to its circulatory function and rely upon a 2D viscoelastic spectrin–F-actin network attached to the membrane. A role for nonmuscle myosin II (NMII) contractility in generating tension in this network and controlling RBC shape has not been tested. We show that NMIIA forms bipolar filaments in RBCs, which associate with F-actin at the membrane. NMIIA motor activity regulates interactions with the spectrin–F-actin network to control RBC biconcave shape and deformability. These results provide a previously undescribed mechanism for actomyosin force generation at the plasma membrane, and may apply to spectrin–F-actin–based membrane skeleton networks in other cell types, such as neurons and polarized epithelial cells.