Background: Elevated circulating carbohydrate antigen 125 (CA125) is a marker of congestion and a predictor of outcomes in acute heart failure (HF). Less is known about CA125 in chronic ambulatory HF with reduced ejection fraction. Objectives: This study examined the association between baseline CA125 (and changes in CA125) and outcomes in patients with HF with reduced ejection fraction in the DAPA-HF (Dapagliflozin and Prevention of Adverse Outcomes in Heart Failure; NCT03036124) trial and its relationship with the effect of dapagliflozin. Methods: The primary outcome was a composite of a first episode of worsening HF or cardiovascular death. CA125 was measured at baseline and 12 months following randomization. Results: Median baseline CA125 was 13.04 U/mL (IQR: 8.78-21.13 U/mL) in 3,123 of 4,774 patients with available data. Compared with CA125 ≤35 U/mL (upper limit of normal), patients with CA125 >35 U/mL were at a higher risk of the primary outcome (adjusted HR: 1.59; 95% CI: 1.29-1.96). The adjusted risks of the primary outcome relative to quartile 1 (Q1) (≤8.78 U/mL) were as follow: Q2, 8.79-13.04 U/mL (HR: 0.94; 95% CI: 0.71-1.24); Q3, 13.05-21.13 U/mL (HR: 1.22; 95% CI: 0.94-1.59); Q4, ≥21.14 U/mL (HR: 1.63; 95% CI: 1.28-2.09). The beneficial effect of dapagliflozin compared with placebo on the primary outcome was consistent whether CA125 was analyzed in quartiles (interaction P = 0.13) or as a continuous variable (interaction P = 0.75). The placebo-corrected relative change in CA125 at 12 months was −5.2% (95% CI: −10.6% to 0.5%; P = 0.07). Conclusions: In DAPA-HF, elevated CA125 levels were an independent predictor of the risk of worsening HF or cardiovascular death. Dapagliflozin reduced the risk of worsening HF or cardiovascular death regardless of baseline CA125.