A new dimeric sesquiterpenoid with eudesmane and aromadendrane skeletons linked through a urea fragment, halichonadin E ( 1), was isolated from a marine sponge Halichondria sp., and the gross structure and relative configuration of 1 were elucidated on the basis of spectroscopic data. Halichonadin E ( 1) is the first hetero-dimeric sesquiterpenoid with eudesmane and aromadendrane skeletons linked through a urea fragment.