The first chemical study of the common species Terrazoanthus onoi, present off the coast of Ecuador, led to the identification of a new family of 2-aminoimidazole alkaloids named terrazoanthines A-C (1-3). Homologues 1 and 2 feature an unprecedented 6-(imidazol-5-yl)benzo[d]imidazole. Acyl substitution pattern and complete configurational assignments were deduced from comparison between experimental and theoretical (13)C NMR and ECD data, respectively. These compounds may represent key derivatives in the biosynthesis of zoanthoxanthins.