A new 1-imidazoyl-3-carboxy-6-hydroxy-beta-carboline alkaloid, named hyrtiocarboline (1), was isolated from a Papua New Guinea marine sponge, Hyrtios reticulatus. The structure was elucidated from spectroscopic data, including (1)H-(15)N HMBC NMR experiments, which provided complementary (15)N chemical shift information in support of the structure. This compound showed selective antiproliferative activity against H522-T1 non-small cell lung, MDA-MB-435 melanoma, and U937 lymphoma cancer cell lines.