Aphananoid A, a limonoid which features a rare C24 appendage and new 5/6/5 fused-ring framework, was obtained from Aphanamixis polystachya. The planar structure as well as the absolute configuration was identified based on extensive spectroscopic analysis and electronic circular dichroism calculations. The biogenetic pathway of aphananoid A was also speculated, which arises from the triterpene by the 3,4-seco-7,8-seco-6,8 cyclo-7,30-decarbon key pattern. In addition, bioassays indicated that aphananoid A inhibited NO production in the RAW264.7 cell line (46.80 ± 1.93%).