A compound that inhibits the formation of a complex composed of the ubiquitin E2 enzyme Ubc13 and Uev1A was isolated from the marine sponge Leucetta aff. microrhaphis. The compound was identified as leucettamol A (1) by spectroscopic analysis. Its inhibition of Ubc13-Uev1A interaction was tested by the ELISA method, revealing an IC(50) value of 50 microg/mL. The compound is the first inhibitor of Ubc13-Uev1A interaction, that is, that of the E2 activity of Ubc13. Such inhibitors are presumed to be leads for anti-cancer agents that upregulate activity of the tumor suppressor p53 protein. Interestingly, hydrogenation of 1 increased its inhibitory activity with an IC(50) value of 4 microg/mL, while its tetraacetate derivative was inactive, indicating that the hydroxy and/or amino groups of 1 are required for the inhibition.