The electronic and magnetic properties of nanographenes strongly depend on their size, shape and topology. While many nanographenes present a closed shell electronic structure, certain molecular topologies may lead to an open-shell structure. Triangular-shaped nanographenes with zigzag edges, which exist as neutral radicals, are of considerable interest both in fundamental science and for future technologies aimed at harnessing their intrinsic high-spin magnetic ground states for spin based operations and information storage. Their synthesis, however, is extremely challenging owing to the presence of unpaired electrons, which confers them with enhanced reactivity. We report a combined in-solution and on surface synthesis of pi-extended triangulene, a non-Kekule nanographene with the structural formula C(33)H(1)s, consisting of ten benzene rings fused in a triangular fashion. The distinctive topology of the molecule entails the presence of three unpaired electrons that couple to form a spin quartet ground state. The structure of individual molecules adsorbed on an inert gold surface is confirmed through ultrahigh-resolution scanning tunneling microscopy. The electronic properties are studied via scanning tunneling spectroscopy, wherein unambiguous spectroscopic signatures of the spin-split singly occupied molecular orbitals are found. Detailed insight into its properties is obtained through tight-binding, density functional and many-body perturbation theory calculations, with the latter providing evidence that pi-extended triangulene retains its open-shell quartet ground state on the surface. Our work provides unprecedented access to open-shell nanographenes with high-spin ground states, potentially useful in carbon-based spintronics.