A six-step one-pot reaction was designed for synthesizing homodimeric 7-phenylindolo[3,2-a]carbazoles from 1H-indoles and β-nitrostyrenes, in the presence of SnCl₂·2H₂O. The reactions proceeded under very mild conditions and the desired heterocycles were obtained in moderate to good yields. An unprecedented mechanism involving sequential indole dimerization, regioselective nucleophilic conjugate addition of the resulting 2,3′-biindole to β-nitrostyrene and formal intramolecular [4 + 2]-cycloaddition is proposed.