Background The incidence and implications of worsening renal function (WRF) after mitral valve transcatheter edge-to-edge repair (TEER) in patients with heart failure (HF) are unknown. Therefore, the aim of this study was to determine the proportion of patients with HF and secondary mitral regurgitation who develop persistent WRF within 30 days following TEER, and whether this development portends a worse prognosis. Methods and Results In the COAPT (Cardiovascular Outcomes Assessment of the MitraClip Percutaneous Therapy for Heart Failure Patients With Functional Mitral Regurgitation) trial, 614 patients with HF and severe secondary mitral regurgitation were randomized to TEER with the MitraClip plus guideline-directed medical therapy (GDMT) versus GDMT alone. WRF was defined as serum creatinine increase ≥1.5× or ≥0.3 mg/dL from baseline persisting to day 30 or requiring renal replacement therapy. All-cause death and HF hospitalization rates between 30 days and 2 years were compared in patients with and without WRF. WRF at 30 days was present in 11.3% of patients (9.7% in the TEER plus GDMT group and 13.1% in the GDMT alone group; P=0.23). WRF was associated with all-cause death (hazard ratio [HR], 1.98 [95% CI, 1.3-3.03]; P=0.001) but not HF hospitalization (HR, 1.47 [ 95% CI, 0.97-2.24]; P=0.07) between 30 days and 2 years. Compared with GDMT alone, TEER reduced both death and HF hospitalization consistently in patients with and without WRF (Pinteraction=0.53 and 0.57, respectively). Conclusions Among patients with HF and severe secondary mitral regurgitation, the incidence of WRF at 30 days was not increased after TEER compared with GDMT alone. WRF was associated with greater 2-year mortality but did not attenuate the treatment benefits of TEER in reducing death and HF hospitalization compared with GDMT alone. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT01626079.