A new biflavanone, 2,2'',3,3''-tetrahydrorobustaflavone 7,4',7''-trimethyl ether (1) was isolated from the whole plant of Selaginella doederleinii HIERON. (Selaginellaceae) together with the known biflavonoid, robustaflavone 7,4',7''-trimethyl ether (2) as the cytotoxic constituents against the three human cancer cell lines, HCT, NCI-H358, and K562. The structure of the new compound 1 was elucidated by spectral analysis including various 1D- and 2D-NMR experiments.