The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open-shell character has so far been missing. Here, we present the on-surface synthesis of tridecacene molecule by thermal annealing of octahydrotridecacene on Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of the inelastic signal at 126 meV. We attribute the inelastic signal to the spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carry out many-body ab-initio calculations as well as model Hamiltonian to take into account the effect of metallic substrate. Moreover, we provide a detailed analysis of how the dynamical electron correlation and virtual charge fluctuation between molecule and metallic surface reduces the singlet-triplet band gap. Therefore, this work provides the first experimental confirmation of the magnetic character of tridecacene.