In the search for potential anticancer compounds from marine organisms, a new N-oxime bromotyrosine alkaloid, (+)-aplysamine 7 (1), was isolated as a minor component from the Australian sponge Pseudoceratina verrucosa. The planar structure of 1 was characterized by NMR and MS analysis. Its absolute configuration was determined by a synthetic route. The natural product was converted to the (+)- oxime-protected aplysamine 7 (2). Two enantiomers of the oxime-protected aplysamine 7 (3 and 4) were synthesized from O-methyl-L-tyrosine and (±)-octopamine. A comparison between the specific rotation of 2 with those of 3 and 4 resulted in the assignment of an S-configuration for aplysamine 7. This natural product showed selective potency against PC3 prostate cancer cells with an IC50 of 4.9 µM.