The creation of increasingly large multipartite entangled states is not only a fundamental scientific endeavour in itself^1-3, but is also the enabling technology for quantum information^4,5. Tremendous experimental effort has been devoted to generating multiparticle entanglement with a growing number of qubits^6-16. So far, up to six spatially separated single photons^10-14 have been entangled based on parametric downconversion¹⁷. Multiple degrees of freedom of a single photon have been exploited to generate forms of hyper-entangled states^18,19. Here, using new ultra-bright sources of entangled photon pairs²⁰, an eight-photon interferometer and post-selection detection, we demonstrate for the first time the creation of an eight-photon Schrödinger cat state ¹ with genuine multipartite entanglement. The ability to control eight individual photons represents a step towards optical quantum computation, and will enable new experiments on, for example, quantum simulation ^21,22, topological error correction²³ and testing entanglement dynamics under decoherence²⁴.