Thirteen new grayanane diterpenoids (1-13) and 15 known analogues (14-28) were isolated from a leaf extract of Pieris japonica. Their structures were determined by spectrometric and spectroscopic methods, including HRESIMS, NMR, IR, and UV. The absolute configurations of 1, 3, 7-9, and 16 were defined by single-crystal X-ray diffraction analysis. 17-Hydroxygrayanotoxin XIX (1) represents the first example of a 17-hydroxygrayan-15(16)-ene diterpenoid. Diterpenoids 1-28 were evaluated for their antinociceptive activities, and 4, 9, 13, 21, and 26-28 displayed significant antinociceptive activities at a dose of 5.0 mg/kg (ip) in the HOAc-induced writhing test in mice. 17-Hydroxygrayanotoxin XIX (1) exhibited potent antinociceptive effects with writhe inhibition rates of 56.3% and 64.8% at doses of 0.04 and 0.2 mg/kg, respectively, which were almost equivalent to the positive control, morphine. Rhodomollein X (26) and rhodojaponin VI (27) showed more potent antinociceptive effects than morphine at doses of 0.04 and 0.2 mg/kg. A preliminary structure-activity relationship for the antinociceptive effects of diterpenoids 1-28 is discussed.