Plakinidine D (4), a new pyrroloacridine metabolite, was isolated together with the known compound 3,5-diiodo-4-methoxyphenethylamine (5) from the ascidian Didemnum rubeum collected near the island of Rota, Northern Mariana Islands. Spectroscopic methods were used to determine the structure of plakinidine D, which was also confirmed through the formation and characterization of the derivatives N-acetylplakinidine D and 11-deoxyplakinidine D. Plakinidine D represents the first metabolite in the pyrroloacridine family of compounds to be isolated from an ascidian.