Cell surface area rapidly increases during mechanical and hypoosmotic stresses. Such expansion of the plasma membrane requires 'membrane reservoirs' that provide surface area and buffer membrane tension, but the sources of this membrane remain poorly understood. In principle, the flattening of invaginations and buds within the plasma membrane could provide this additional surface area, as recently shown for caveolae in animal cells. Here, we used microfluidics to study the rapid expansion of the yeast plasma membrane in protoplasts, which lack the rigid cell wall. To survive hypoosmotic stress, yeast cell protoplasts required eisosomes, protein-based structures that generate long invaginations at the plasma membrane. Both budding yeast and fission yeast protoplasts lacking eisosomes were unable to expand like wild-type protoplasts during hypoosmotic stress, and subsequently lysed. By performing quantitative fluorescence microscopy on single protoplasts, we also found that eisosomes disassembled as surface area increased. During this process, invaginations generated by eisosomes at the plasma membrane became flattened, as visualized by scanning electron microscopy. We propose that eisosomes serve as tension-dependent membrane reservoirs for expansion of yeast cells in an analogous manner to caveolae in animal cells.