A single photon does not naturally decay into smaller energy photons, unlike other strongly interacting particles such as electrons in solids. However, using a carefully engineered superconducting circuit and a detection system that employs a quantum-limited amplifier, we successfully recorded the direct decay of one photon into three photons, each carrying one-third of the original photon’s energy. In this process, the interaction between photons is facilitated by a strongly nonlinear superconducting qubit—a small Josephson junction—that connects two high-impedance resonators. Notably, the generation of these photon triplets does not require any strong classical drive or parametric pumping scheme. This novel regime of light-matter interaction opens exciting possibilities for the emerging field of many-body quantum optics and reveals new loss mechanisms. Once fully understood, these mechanisms could lead to improvements in superconducting qubits.