We demonstrate that suspended single-crystal GaAs nanomembranes exhibit mechanical Q-factors exceeding 2 × 106 at room temperature, which makes them a very promising platform for optomechanics. Because of the completely removed substrate and their millimeter-scale lateral size, the membranes can be incorporated in macroscopic optical cavities for quantum optomechanics experiments. We have measured the mechanical mode spectrum and spatial profiles and find good agreements with theory. Our work paves the way for optomechanical experiments with direct band gap semiconductors in which not only radiation pressure but also other mechanisms involving embedded light emitters could be exploited for quantum optical control of massive mechanical systems. © 2011 American Institute of Physics.