Mantidisflavin A (1) was isolated from the egg cases of the Tenodera sinensis Saussure. It exhibits an unprecedented 6/6/6/5/5 skeleton, accompanied by the formation of two additional fused heterocycles through a novel C-C bond and an oxygen bridge on riboflavin backbone. The structure of 1 was identified by spectroscopic and computational methods. To assess the inhibitory effect against renal fibrosis, compound 1 was evaluated in TGF-β1-induced rat kidney epithelial cells, and the results were compared to those of its precursor, riboflavin.