Superposition, entanglement and non-locality constitute fundamental features of quantum physics. The fact that quantum physics does not follow the principle of local causality1–3can be experimentally demonstrated in Bell tests4performed on pairs of spatially separated, entangled quantum systems. Although Bell tests, which are widely regarded as a litmus test of quantum physics, have been explored using a broad range of quantum systems over the past 50 years, only relatively recently have experiments free of so-called loopholes5succeeded. Such experiments have been performed with spins in nitrogen–vacancy centres6, optical photons7–9and neutral atoms10. Here we demonstrate a loophole-free violation of Bell’s inequality with superconducting circuits, which are a prime contender for realizing quantum computing technology11. To evaluate a Clauser–Horne–Shimony–Holt-type Bell inequality4, we deterministically entangle a pair of qubits12and perform fast and high-fidelity measurements13along randomly chosen bases on the qubits connected through a cryogenic link14spanning a distance of 30 metres. Evaluating more than 1 million experimental trials, we find an averageSvalue of 2.0747 ± 0.0033, violating Bell’s inequality with aPvalue smaller than 10−108. Our work demonstrates that non-locality is a viable new resource in quantum information technology realized with superconducting circuits with potential applications in quantum communication, quantum computing and fundamental physics15.