Importance Treatment with finerenone, a nonsteroidal mineralocorticoid receptor antagonist (MRA), improved outcomes in patients with heart failure with mildly reduced or preserved ejection fraction in FINEARTS-HF, but was associated with increased levels of serum potassium in follow-up. Objective To investigate the frequency and predictors of serum potassium level greater than 5.5 mmol/L and less than 3.5 mmol/L and examine the treatment effect associated with finerenone, relative to placebo, on clinical outcomes based on postrandomization potassium levels. Design, Setting, and Participants Secondary analysis of the FINEARTS-HF multicenter, randomized clinical trial, performed between September 14, 2020, and January 10, 2023, with a median follow-up of 32 months (final date of follow-up: June 14, 2024). Patients with heart failure and left ventricular ejection fraction greater than or equal to 40%, New York Heart Association class II to IV symptoms, and elevated natriuretic peptides were included. Intervention Participants received finerenone or placebo. Main Outcomes and Measures The primary outcome was a composite of total worsening heart failure events or cardiovascular death. Results A total of 6001 participants were included (3003 randomized to receive finerenone and 2998 randomized to receive placebo). The increase in serum potassium was greater in the finerenone group than the placebo group at 1 month (median [IQR] difference, 0.19 [0.17-0.21] mmol/L) and 3 months (median [IQR] difference, 0.23 [0.21-0.25] mmol/L), which persisted for the remainder of trial follow-up. Finerenone increased the risks of potassium level increasing to greater than 5.5 mmol/L (hazard ratio [HR], 2.16 [95% CI, 1.83-2.56]; P < .001) and decreased the risks for potassium level decreasing to less than 3.5 mmol/L (HR, 0.46 [95% CI, 0.38-0.56]; P < .001). Both low (< 3.5 mmol/L; HR, 2.49 [95% CI, 1.8-3.43]) and high (>5.5 mmol/L; HR, 1.64 [95% CI, 1.04-2.58]) potassium levels were associated with higher subsequent risks of the primary outcome in both treatment groups. Nevertheless, the risk of the primary outcome was generally lower in patients treated with finerenone compared with placebo, even in those whose potassium level increased to greater than 5.5 mmol/L. Conclusions and Relevance In patients with heart failure with mildly reduced or preserved ejection fraction, finerenone resulted in more frequent hyperkalemia and less frequent hypokalemia. However, with protocol-directed surveillance and dose adjustment, clinical benefit associated with finerenone relative to placebo was maintained even in those whose potassium level increased to greater than 5.5 mmol/L. Trial Registration ClinicalTrials.gov Identifier: NCT04435626