Key Points Simultaneous heart–kidney transplant is associated with high rates of kidney graft failure which are worse when compared with kidney transplant alone.The major causes of kidney graft failure in simultaneous heart–kidney transplant recipients were patient death and primary nonfunction of kidney graft. Background The indications and outcomes of simultaneous heart–kidney transplantation (SHKT) remain suboptimally defined. Risk factors for renal graft failure after SHKT also remain poorly defined. Methods We analyzed the renal graft outcomes among SHKT recipients using United Network for Organ Sharing database from 2015 to 2020. To evaluate for factors associated with poor renal outcomes, we compared SHKT and kidney transplantation alone recipients using propensity score matching. Results Among SHKT recipients, the rate of primary nonfunction (PNF) of kidney graft was 3%, the 30-day kidney graft failure rate was 7.0%, and the 30-day post-transplant mortality rate was 4.1%. The incidence of kidney delayed graft function was 27.5%. Kidney graft failure was seen early post-SHKT with most common causes of patient death (43.9%) and PNF of kidney graft (41.5%). One- and 2-year patient survival was 89.2% and 86.5%, and 1- and 2-year freedom from kidney graft failure was 85.4% and 82.7%, respectively. In subgroup analysis of SHKT recipients, use of pretransplant mechanical cardiac support (adjusted odds ratio [aOR], 2.57; P = 0.017), higher calculated panel reactive antibody (aOR, 1.76; P = 0.016), and older donor age per 10 years (aOR, 1.94; P = 0.001) were associated with PNF. Pretransplant extracorporeal membrane oxygenation support was associated with the increased risk of 30-day recipient mortality (aOR, 5.55; P = 0.002). Increased 30-day graft failure was seen in SHKT recipients with pretransplant mechanical cardiac support (aOR, 1.77; P = 0.038) and dialysis at the time of transplant (aOR, 1.72; P = 0.044). Multivariable Cox hazard analysis demonstrated that SHKT, when compared with kidney transplantation alone, is associated with increased kidney graft failure (hazard ratio, 2.56; P < 0.001) and recipient mortality (hazard ratio, 2.65; P < 0.001). Conclusions SHKT is associated with high rates of kidney graft failure. Identification of risk factors of renal graft failure can help optimize recipient selection for SHKT versus kidney after heart transplantation, especially after introduction of the new safety-net policy.