Beginning with the early work of Clar et al. in 1955, zethrenes and their laterally extended homologues, super-zethrenes, have been intensively studied in the solution phase and widely investigated as optical and charge transport materials. Superzethrenes are also considered to exhibit an open-shell ground state and may thus serve as model compounds to investigate nanoscale π-magnetism. However, their synthesis is extremely challenging due to their high reactivity. We report here the on-surface synthesis of the hitherto largest zethrene homologue - super-nonazethrene - on Au(111). Using single-molecule scanning tunneling microscopy and spectroscopy, we show that super-nonazethrene exhibits an open-shell singlet ground state featuring a large spin polarization-driven electronic gap of 1 eV. Consistent with the emergence of an open-shell ground state, high-resolution tunneling spectroscopy reveals singlet-triplet spin excitations in super-nonazethrene, characterized by a strong intramolecular magnetic exchange coupling of 51 meV. Given the paucity of zethrene chemistry on surfaces, our results therefore provide unprecedented access to large, open-shell zethrene compounds amenable to scanning probe measurements, with potential application in molecular spintronics.