Background: Circulating high-sensitivity cardiac troponin T (hsTnT) predominantly reflects myocardial injury, and higher levels are associated with a higher risk of worsening heart failure and death in patients with heart failure with reduced ejection fraction. Less is known about the prognostic significance of changes in hsTnT over time, the effects of dapagliflozin on clinical outcomes in relation to baseline hsTnT levels, and the effect of dapagliflozin on hsTnT levels. Methods: DAPA-HF (Dapagliflozin and Prevention of Adverse Outcomes in Heart Failure) was a randomized, double-blind, placebo-controlled trial of dapagliflozin (10 mg daily) in patients with New York Heart Association class II to IV symptoms and left ventricular ejection fraction ≤40% (median follow-up, 18.2 months). hsTnT (Roche Diagnostics) was measured at baseline in 3112 patients and at 1 year in 2506 patients. The primary end point was adjudicated worsening heart failure or cardiovascular death. Clinical end points were analyzed according to baseline hsTnT and change in hsTnT from baseline to 1 year. Comparative treatment effects on clinical end points with dapagliflozin versus placebo were assessed by baseline hsTnT. The effect of dapagliflozin on hsTnT was explored. Results: Median baseline hsTnT concentration was 20.0 (25th–75th percentile, 13.7–30.2) ng/L. Over 1 year, 67.9% of patients had a ≥10% relative increase or decrease in hsTnT concentrations, and 43.5% had a ≥20% relative change. A stepwise gradient of higher risk for the primary end point was observed across increasing quartiles of baseline hsTnT concentration (adjusted hazard ratio Q4 versus Q1, 3.44 [95% CI, 2.46–4.82]). Relative and absolute increases in hsTnT over 1 year were associated with higher subsequent risk of the primary end point. The relative reduction in the primary end point with dapagliflozin was consistent across quartiles of baseline hsTnT ( P -interaction=0.55), but patients in the top quartile tended to have the greatest absolute risk reduction (absolute risk difference, 7.5% [95% CI, 1.0%–14.0%]). Dapagliflozin tended to attenuate the increase in hsTnT over time compared with placebo (relative least squares mean reduction, –3% [–6% to 0%]; P =0.076). Conclusions: Higher baseline hsTnT and greater increase in hsTnT over 1 year are associated with worse clinical outcomes. Dapagliflozin consistently reduced the risk of the primary end point, irrespective of baseline hsTnT levels. Registration: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT03036124.