Forrestiacids C (1) and D (2), a pair of C-25 epimeric triterpene–diterpene adducts were isolated from the needles and twigs of the vulnerable conifer Pseudotsuga forrestii. This unprecedented class of compounds might be generated via an intermolecular Michael addition reaction of a rearranged 6/6/5/5-fused spiro-lanostene with an abietene. Their structures were established by spectroscopic data and X-ray crystallography. The adducts showed inhibitory activities against the ATP-citrate lyase (ACL) and acetyl-CoA carboxylase 1 (ACC1), two rate-limiting enzymes in the de novo lipogenesis pathway.