A new alkaloid, 5-bromo-8-methoxy-1-methyl-β-carboline (1), has been isolated from the New Zealand marine bryozoan Pterocella vesiculosa. Structural elucidation was achieved through NMR spectroscopic and mass spectrometric analysis, and a single-crystal X-ray diffraction analysis of 1 was performed. The biological activity of 1 was assessed against P388 murine leukemia cells and three microorganisms and compared with that of a number of related β-carboline alkaloids.