A new bisindole, caulersin (4), was isolated from the alga Caulerpa serrulata collected from the Xisha Islands in the South China Sea. The alga was extracted with ethanol, and the extract was partitioned between ethyl acetate and water. The organic fraction was purified by silica gel chromatography to yield caulersin (4). Its structure was established using spectroscopic techniques including HREIMS, NMR, IR, and UV. The molecular formula C21H14N2O3 was determined from HREIMS (m/z 342.0992 [M+], calculated 342.1004). Unlike the known bisindole caulerpin (1), which has C2 symmetry and 11 13C NMR signals, caulersin (4) showed 21 13C NMR signals, indicating all carbons are distinct. UV data suggested an extensively conjugated aromatic system, and IR spectra exhibited carbonyl absorption bands at 1690 and 1665 cm-1. 1H NMR revealed two hydrogen-bonded NH protons at δ 12.40 and 13.14. Correlations from 1H-1H COSY and HMBC spectra supported the bisindole structure with a seven-membered ring. Caulersin (4) represents a new member of the bisindole class from the genus Caulerpa.