Conventional, hadronic matter consists of baryons and mesons made of three quarks and a quark–antiquark pair, respectively1,21,2. Here, we report the observation of a hadronic state containing four quarks in the Large Hadron Collider beauty experiment. This so-called tetraquark contains two charm quarks, a u‾u and a d‾d quark. This exotic state has a mass of approximately 3,875 MeV and manifests as a narrow peak in the mass spectrum of D00D00π++ mesons just below the D∗+∗+D00 mass threshold. The near-threshold mass together with the narrow width reveals the resonance nature of the state.