Constitutively active G protein a subunits cause cancer, cholera, Sturge-Weber syndrome, and other disorders. Therapeutic intervention by targeted inhibition of constitutively active Ga subunits in these disorders has yet to be achieved. We found that constitutively active Gaq in uveal melanoma (UM) cells was inhibited by the cyclic depsipeptide FR900359 (FR). FR allosterically inhibited guanosine diphosphate-for-guanosine triphosphate (GDP/GTP) exchange to trap constitutively active Gaq in inactive, GDP-bound Gabg heterotrimers. Allosteric inhibition of other Ga subunits was achieved by the introduction of an FR-binding site. In UM cells driven by constitutively active Gaq, FR inhibited second messenger signaling, arrested cell proliferation, reinstated melanocytic differentiation, and stimulated apoptosis. In contrast, FR had no effect on BRAF-driven UM cells. FR promoted UM cell differentiation by reactivating polycomb repressive complex 2 (PRC2)-mediated gene silencing, a heretofore unrecognized effector system of constitutively active Gaq in UM. Constitutively active Gaq and PRC2 therefore provide therapeutic targets for UM. The development of FR analogs specific for other Ga subunit subtypes may provide novel therapeutic approaches for diseases driven by constitutively active Ga subunits or multiple G protein-coupled receptors (GPCRs) where targeting a single receptor is ineffective.