Background: Left ventricular ejection fraction (EF) is an indicator of cardiac function, usually assessed in individuals with heart failure and other cardiac conditions. Although family studies indicate that EF has an important genetic component with heritability estimates up to 0.61, to date only 6 EF-associated loci have been reported. Methods: Here, we conducted a genome-wide association study (GWAS) of EF in 26 638 adults from the Genetic Epidemiology Research on Adult Health and Aging and the UK Biobank cohorts. Results: A meta-analysis combining results from Genetic Epidemiology Research on Adult Health and Aging and UK Biobank identified a novel locus: TMEM40 on chromosome 3p25 (rs11719526; β=0.47 and P =3.10×10 −8 ) that replicated in Biobank Japan and confirmed recent findings implicating the BAG3 locus on chromosome 10q26 in EF variation, with the strongest association observed for rs17617337 (β=−0.83 and P =8.24×10 −17 ). Although the minor allele frequencies of TMEM40 rs11719526 were generally common (between 0.13 and 0.44) in different ethnic groups, BAG3 rs17617337 was rare (minor allele frequencies<0.05) in Asian and African ancestry populations. These associations were slightly attenuated, after considering antecedent cardiac conditions (ie, heart failure/cardiomyopathy, hypertension, myocardial infarction, atrial fibrillation, valvular disease, and revascularization procedures). This suggests that the effects of the lead variants at TMEM40 or BAG3 on EF are largely independent of these conditions. Conclusions: In this large and multiethnic study, we identified 2 loci, TMEM40 and BAG3 , associated with EF at a genome-wide significance level. Identifying and understanding the genetic determinants of EF is important to better understand the pathophysiology of this strong correlate of cardiac outcomes and to help target the development of future therapies.