BACKGROUND High resting heart rate (RHR) occurs in parallel with type 2 diabetes (T2D) and metabolic disorders, implying shared etiology between them. However, it is unknown if they are causally related, and no study has been conducted to investigate the shared mechanisms underlying these associations. OBJECTIVES The objective of this study was to understand the genetic basis of the association between resting heart rate and cardiometabolic disorders/T2D. METHODS This study examined the genetic correlation, causality, and shared genetics between RHR and T2D using LD Score regression, generalized summary data-based Mendelian randomization, and transcriptome wide association scan (TWAS) in UK Biobank data (n = 428,250) and summary-level data for T2D (74,124 cases and 824,006 control subjects) and 8 cardiometabolic traits (sample size ranges from 51,750 to 236,231). RESULTS Significant genetic correlation between RHR and T2D (r(g) = 0.22; 95% confidence interval: 0.18 to 0.26; p = 1.99 x 10(-22)), and 6 cardiometabolic traits (fasting insulin, fasting glucose, waist-hip ratio, triglycerides, high-density lipoprotein, and body mass index; r(g) range -0.12 to 0.24; all p < 0.05) were observed. RHR has significant estimated causal effect on T2D (odds ratio: 1.12 per 10-beats/min increment; p = 7.79 x 10(-11)) and weaker causal estimates from T2D to RHR (0.32 beats/min per doubling increment in T2D prevalence; p = 6.14 x 10(-54)). Sensitivity analysis by controlling for the included cardiometabolic traits did not modify the relationship between RHR and T2D. TWAS found locus chr2q23.3 (rs1260326) was highly pleiotropic among RHR, cardiometabolic traits, and T2D, and identified 7 genes (SMARCAD1, RP11-53O19.3, CTC-498M16.4, PDE8B, AKTIP, KDM4B, and TSHZ3) that were statistically independent and shared between RHR and T2D in tissues from the nervous and cardiovascular systems. These shared genes suggested the involvement of epigenetic regulation of energy and glucose metabolism, and AKT activation-related telomere dysfunction and vascular endothelial aging in the shared etiologies between RHR and T2D. Finally, FADS1 was found to be shared among RHR, fasting glucose, high-density lipoprotein, and triglycerides. CONCLUSIONS These findings provide evidence of significant genetic correlations and causation between RHR and T2D/cardiometabolic traits, advance our understanding of RHR, and provide insight into shared etiology for high RHR and T2D. (C) 2019 by the American College of Cardiology Foundation.