Two novel nitrogenous diterpenoids, sarinfacetamides A (1) and B (2), featuring an uncommon tricyclo[6.3.1.01,5]dodecane scaffold, and a known related diterpene (3) were isolated from the South China Sea soft coral Sarcophyton infundibuliforme. Their structures, including the absolute configuration of 1, were established by extensive spectroscopic analysis and TDDFT-ECD calculation. Compounds 1 and 3 exhibited interesting promotion effects on the ConA-induced T lymphocyte proliferation. A plausible biosynthetic pathway for 1 and 2 was also proposed.