The cycle of GTP binding and hydrolysis controls heterotrimeric G proteins, and mutations reducing GTPase activity result in constitutive G protein signaling. In Gαq (gene: GNAQ) such mutations cause uveal melanoma and Sturge-Weber syndrome. Finding pharmacological agents that inhibit Gαq will be beneficial for research with therapeutic potential. Previously discovered bacterial depsipeptides (FR900359 and YM-254890) bind directly to Gαq and stabilize its inactive complex with GDP, but suffer from limitations of distribution and bioavailability. We used the established Gαq-YM-254890 complex structure to dock small-molecule drugs into the depsipeptide binding site of Gαq. Our in silico screen of 5000 Food Drug and Administration-approved, experimental, and withdrawn drugs predicted that thiazolidinediones are potential ligands of Gαq. Analysis of G protein coupled receptor-stimulated G protein GTPγS binding demonstrated that troglitazone (441 Da) inhibited Gq nucleotide exchange with the IC50 of ∼31.7 μM. The thiazolidinedione analogs, rosiglitazone and pioglitazone, had no effect. High concentrations of troglitazone modestly inhibited Gi1 and Gs, but not G13. In G protein thermal stability assays, troglitazone and FR900359 stabilized purified Gαq-GDP, indicating direct binding. Consistent with its negative effect on Gq signaling, in MIN6 mouse insulinoma cells, troglitazone inhibited Ca2+ mobilization, extracellular regulated protein kinase phosphorylation, and insulin secretion stimulated by the Gq-coupled M3 muscarinic cholinergic receptor. Troglitazone and FR900359 inhibited proliferation of MEL92.1 uveal melanoma cells driven by a GNAQ-Q209L driver mutation, but not of SK-MEL-28 cells driven by BRAF-V600E. Together, our study shows that troglitazone may be a promising new lead for the development of a <500 Da small-molecule therapeutic Gαq inhibitor. SIGNIFICANCE STATEMENT: Troglitazone, unlike other thiazolidinediones, directly binds and inhibits activity of heterotrimeric G protein Gq, with a weaker effect on Gi. Troglitazone may find usage as a repurposed drug scaffold to build novel small-molecule Gαq inhibitors with better bioavailability than depsipeptide Gαq inhibitors.