Conventional hadronic matter consists of baryons and mesons made of three quarks and quark-antiquark pairs, respectively. The observation of a new type of hadronic state, a doubly charmed tetraquark containing two charm quarks, an anti-u and an anti-d quark, is reported using data collected by the LHCb experiment at the Large Hadron Collider. This exotic state with a mass of about 3875 MeV/c 2 manifests itself as a narrow peak in the mass spectrum of D0D0π + mesons just below the D∗+D0 mass threshold. The near-threshold mass together with a strikingly narrow width reveals the resonance nature of the state.