A new indole alkaloid, konbamidin, has been isolated from the Okinawan marine sponge Ircinia sp. Its structure was determined by spectral data (eims, hreims, uv, ¹H- and ¹³C-nmr, HMBC) and synthesis. The absolute stereochemistry at C-2 was established as R-configuration by comparison with (R)- and (S)-konbamidin derived from D- and L-tryptophan (via reaction with glyoxylic acid and NaBH₃CN) through spectral data (including [α]ᴅ values) and chiral hplc analysis. Konbamidin is the third example of an indole alkaloid from a sponge of the genus Ircinia. Biogenetically, it may be derived from glycine and 3-indoleacrylic acid (which was isolated together with konbamidin from the sponge). Konbamidin exhibited cytotoxicity against HeLa cells in vitro with an IC₅₀ value of 5.4 μg/ml, while (S)-konbamidin was not significantly cytotoxic.