Two-dimensional van der Waals materials have shown great promise for a variety of electronic, optoelectronic, sensing and energy conversion applications. Since almost every atom in these two-dimensional crystals is exposed to the surface, covalent surface termination could provide a powerful method for the controlled tuning of material properties. Here we demonstrate a facile, one-step metathesis approach that directly converts CaGe₂ crystals into mm-sized crystals of methyl-terminated germanane (GeCH₃). Replacing --H termination in GeH with --CH₃ increases the band gap by ~0.1 eV to 1.7 eV, and produces band edge fluorescence with a quantum yield of ~0.2%, with little dependence on layer thickness. Furthermore, the thermal stability of GeCH₃ has been increased to 250 °C compared with 75 °C for GeH. This one-step metathesis approach should be applicable for accessing new families of two-dimensional van der Waals lattices that feature precise organic terminations and with enhanced optoelectronic properties.