Ovataline (1), which is a polar metabolite containing a hexahydroquinoline moiety, was isolated from cultures of the marine dinoflagellate Ostreopsis cf. ovata. 1 was characterized as a zwitterionic compound with hexahydroquinoline and tetrahydropyran rings. The configurations of the chiral centers in 1 were established using ROESY correlations, J-based configurational and Mosher reaction analyses, and density functional theory calculations. 1 exhibited a 78% (1 μM) inhibition of type II 5α-reductase in testosterone propionate-induced RWPE-1 human prostatic cells.