Thirteen C-methylated flavonoid glycosides (1–13), along with 15 previously known flavonoids (14–28), were isolated from rhizomes of Pentarhizidium orientale. Among these compounds, matteuorienates D–K (1–8) were obtained as analogues of matteuorienates A–C (14–16), which contain a characteristic 3-hydroxy-3-methylglutaryl (HMG) moiety. The structures of 1–13 were characterized by spectroscopic analysis and chemical derivatization. The isolates were evaluated for their antiviral activities against influenza virus (H1N1), with compounds 21, 22, 23, 25, and 26showing inhibitory effects (IC50 of 23.9–30.3 μM) against neuraminidases.