Patients with heart failure with mildly reduced ejection fraction (HFmrEF) and heart failure with preserved ejection fraction (HFpEF) experience a high burden of symptoms, physical limitations, and poor quality of life; improving health status is a key goal of management. In a prespecified analysis of the DELIVER (Dapagliflozin Evaluation to Improve the Lives of Patients With Preserved Ejection Fraction Heart Failure) trial, we examine effects of dapagliflozin on health status using the Kansas City Cardiomyopathy Questionnaire (KCCQ). The DELIVER trial randomized patients with symptomatic HFmrEF/HFpEF to dapagliflozin 10 mg or placebo. KCCQ was evaluated at randomization, 1, 4, and 8 months; KCCQ Total Symptom Score (TSS) was a key secondary endpoint. Patients were stratified by KCCQ-TSS tertiles; Cox models examined effects of dapagliflozin on clinical outcomes. We evaluated the effects of dapagliflozin on KCCQ-TSS, Physical Limitations (PLS), Clinical Summary (CSS), and Overall Summary (OSS) domains. Responder analyses compared proportions of dapagliflozin vs placebo-treated patients with clinically meaningful changes in KCCQ. A total of 5,795 patients had baseline KCCQ (median KCCQ-TSS 72.9). The effects of dapagliflozin on reducing cardiovascular death/worsening HF appeared more pronounced in patients with greater baseline symptom burden (lowest-to-highest KCCQ-TSS tertile: HR: 0.70 [95% CI: 0.58-0.84]; 0.81 [95% CI: 0.65-1.01]; 1.07 [95% CI: 0.83-1.37]; Pinteraction = 0.026). Dapagliflozin improved KCCQ-TSS, -PLS, -CSS, and -OSS at 8 months (2.4, 1.9, 2.3, and 2.1 points higher vs placebo; P < 0.001 for all). Dapagliflozin-treated patients experienced improvements in KCCQ-TSS regardless of EF (Pinteraction = 0.85). Fewer dapagliflozin-treated patients had deterioration, and more had improvements in all KCCQ domains at 8 months. The clinical benefits of dapagliflozin in HFmrEF/HFpEF appear especially pronounced in those with greater baseline symptom impairment. Dapagliflozin improved all KCCQ domains and the proportion of patients experiencing clinically meaningful changes in health status. (Dapagliflozin Evaluation to Improve the LIVEs of Patients With PReserved Ejection Fraction Heart Failure [DELIVER]; NCT03619213).