Acanthodendrilline (1), a new bromotyrosine alkaloid, was isolated from the Thai marine sponge Acanthodendrilla sp. The structure of 1 was fully characterized by spectroscopic analysis, in agreement with the synthesized compound used to resolve the single chiral center at C-11. Total synthesis of the enantiomers of 1 allowed for the comparison of specific rotation values and hence the determination of the absolute configuration as 11-S. Cytotoxicity evaluation revealed that (S)-1 exhibited approximately three-fold more potent cytotoxicity against the human non-small cell lung cancer H292 cell line than (R)-1.