Highly lattice‐mismatched In x Ga 1-x P (x≤0.38) layers were grown on GaP substrates by gas‐source molecular beam epitaxy. A relatively thin, compositionally linear‐graded buffer layer was used to reduce the number of threading dislocations. Studies by double‐crystal x‐ray diffraction and transmission electron microscopy show this buffer layer to be 97% strain‐relaxed along both 〈110〉 directions with dislocations well confined within the graded buffer and the substrate. Threading dislocation densities in the top layers were less than 1×10⁷ cm^-2. Room‐temperature photoluminescence, ranging from 560 to 600 nm, is achieved. Heterojunction p‐i‐n diodes emitting at 560 nm at 300 K exhibit good rectifying and reverse breakdown characteristics.