The Josephson effect describes supercurrent flowing through a junction connecting two superconducting leads by a thin barrier 1 . This current is driven by a superconducting phase difference φ between the leads. In the presence of chiral and time-reversal symmetry of the Cooper pair tunnelling process 2 , the current is strictly zero when φ vanishes. Only if these underlying symmetries are broken can the supercurrent for φ = 0 be finite 3–5 . This corresponds to a ground state of the junction being offset by a phase φ 0 , different from 0 or π . Here, we report such a Josephson φ 0 -junction based on a nanowire quantum dot. We use a quantum interferometer device to investigate phase offsets and demonstrate that φ 0 can be controlled by electrostatic gating. Our results may have far-reaching implications for superconducting flux- and phase-defined quantum bits as well as for exploring topological superconductivity in quantum dot systems.