Abstract Most aldosterone-producing adenomas (APA) have gain-of-function somatic mutations of ion channels or transporters. However, their frequency in aldosterone-producing cell-clusters of normal adrenals suggests the existence of co-driver mutations which influence the development or phenotype of APAs. Gain-of-function mutations in both CTNNB1 and GNA11 were found by whole exome sequencing in 3 of 41 APAs from a UK/Irish cohort. Targeted sequencing for exon 3 mutations of CTNNB1 and p.Gln209 mutations of either GNA11 or closely homologous GNAQ confirmed these and 7 further double mutant APAs in this discovery cohort. The presence of GNA11/Q p.Gln209 mutations in CTNNB1 mutant APAs were replicated in 2 cohorts from France (n=14) and Sweden (n=3). In total, 16 (59%) of the 27 CTNNB1 mutant APAs investigated had a mutation at p.Gln209 of GNA11 (n=11) or GNAQ (n=5). Interestingly, CTNNB1-mutant APAs were more commonly present in women (23/27), and of these, those with GNA11/Q mutations were all women except for a pubertal boy. To also note, 9 of 10 of the UK/Irish double mutant APAs in the discovery cohort presented in puberty, pregnancy, or menopause. Mutation of p.Gln209, or homologous p.Gln in GNAS, GNA12-14, impair hydrogen bonds between G-protein α and β subunits. Transfection of H295R cells, an immortalised adrenocortical cell line heterozygous for the p.Ser45Pro mutation of CTNNB1 but wild-type for GNA11-14/Q/S, by each of the GNA11/Q mutations increased aldosterone secretion and CYP11B2 expression (encoding aldosterone synthase) by 1.93-6.1-fold and 8.0-9.8-fold respectively, compared to vector or wild-type -transfected cells. In ZG, GNA11/Q mediate the aldosterone response to angiotensin II, via stimulation of intracellular Ca2+ release by inositol trisphosphate. In the mutant-transfected cells, the stimulatory effect of angiotensin II 10 nM was retained. In order to determine whether the p.Gln209 mutations stimulate aldosterone production even in the absence of CTNNB1 activation, the transfections of H295R cells were repeated after either 24-h treatment with the CTNNB1 inhibitor, ICG-001, or silencing of CTNNB1 using the ONTARGETplus SMARTpool SiRNAs (Dharmacon). Both interventions reduced the aldosterone production relative to vehicle/control-treated cells; however neither ICG-001 nor silencing of CTNNB1 blunted the fold-increase in aldosterone secretion seen in mutant-transfected cells compared to wild-type. In summary, we report the discovery of gain-of-function mutations of the G-protein, GNA11, or its close homologue, GNAQ, in multiple APAs which majority presented during periods of high LH/HCG. To date, the mutation is always residue p.Gln209, and associated with a gain-of-function mutation of CTNNB1. These GNA11/Q p.Gln209 mutations increase aldosterone and CYP11B2 production both in the presence and in the absence of CTNNB1 activation.