Classical antenna theory establishes that there are no coherent isotropic radiators. Thermal radiators can be isotropic, but they lack temporal coherence. Here, we demonstrate that quantum interference processes enable the design of isotropic and unpolarized single-photon sources that preserve temporal coherence properties. We illustrate how this effect can be realized with relatively simple multilevel emitters with degenerate ground states, and we discuss potential implementations based on atomic and solid-state single-photon sources.