A new modified abietane diterpenoid, (3S,4S,5R,10S)-18(4→3)-abeo-3,4,12,18-tetrahydroxy-8,11,13-abietatrien-7-one (1), and two novel dimers, selaginedorffones A (2) and B (3), featuring a new cyclohexene moiety that was biogenetically constructed from two modified abietane diterpenoids through a Diels-Alder reaction were obtained from a methanolic extract of Selaginella moellendorffii, a traditional Chinese herb. The structures of 1-3 were identified by a combination of NMR spectroscopic analysis and ECD calculations. In the present study, diterpenoids were identified from S. moellendorffii for the first time, which supports the presence of diterpene synthases in this plant. These three diterpenoids (1-3) were evaluated for their growth-inhibitory activities against several human cancer cell lines. Of these substances, selaginedorffone B (3) showed cytotoxicity against the MCF-7 human-breast-cancer-cell line (IC50 9.0 μM).