An ensemble of spin defect centers or other atom-like quantum systems in a solid-state host can be used as a compact alternative for an atomic clock thanks to an architecture that overcomes magnetic and temperature-induced systematics. A polariton-stabilized solid-state spin clock hybridizes a microwave resonator with a magnetic-field-insensitive spin transition within the ground state of a spin defect center (e.g., a nitrogen vacancy center in diamond). Detailed numerical and analytical modeling of this polariton-stabilized solid-state spin clock indicates a potential fractional frequency instability below 10−13 over a 1-second measurement time, assuming present-day experimental parameters. This stability is a significant improvement over the state-of-the-art in miniaturized atomic vapor clocks.