Guided by LC-MS/MS molecular networking-based metabolomics and cytotoxic activity, two new discorhabdin-type alkaloids, tridiscorhabdin (1) and didiscorhabdin (2), were isolated from the sponge Latrunculia biformis, collected from the Weddell Sea (Antarctica) at -291 m depth. Their structures were established by HRESIMS, NMR, [α]D, and ECD data coupled with DFT calculations. Both compounds bear a novel C-N bridge (C-1/N-13) between discorhabdin monomers, and 1 represents the first trimeric discorhabdin molecule isolated from Nature. Tridiscorhabdin (1) exhibited strong cytotoxic activity against the human colon cancer cell line HCT-116 (IC50 value 0.31 μM). Copyright © 2020 American Chemical Society and American Society of Pharmacognosy.