An iboga‐vobasine bisindole alkaloid, 16’‐decarbomethoxydihydrovoacamine (1) isolated from the stem bark extract of Tabernaemontana corymbosa exhibited strong cytotoxicity against colorectal cancer cell lines in our preliminary study. As an initial step to elucidate its anti‐proliferative mechanism, the DNA binding and topoisomerase I (topo1) inhibitory activities of the bisindole 1 were investigated. The results of the in vitro DNA binding studies and molecular docking calculations collectively suggested that 1 binds to the DNA minor groove via hydrophobic interactions with a binding constant (K_b) of 7.40×10⁵ M⁻¹. Treatment of Escherichia coli topo1 with 1 did not inhibit the enzyme from relaxing the supercoiled plasmid DNA pBR322, indicating that the cytotoxicity of 1 in colorectal cancer cells is independent of topo1. The bisindole is predicted to possess substantial bioavailability without major toxicity. Moreover, the cytotoxicity of 1 is selective towards the colorectal cancer cells as evaluated using the MTT assay.