Background: Myocardial fibrosis may contribute to the pathophysiology of heart failure with preserved ejection fraction. Given the biochemical targets of sacubitril/valsartan, this study hypothesized that circulating biomarkers reflecting the mechanisms that determine extracellular matrix homeostasis are altered by sacubitril/valsartan compared with valsartan alone. Objectives: This study investigated the effects of sacubitril/valsartan on biomarkers of extracellular matrix homeostasis and the association between biomarkers and the primary endpoint (total heart failure hospitalizations and cardiovascular death). Methods: N-terminal propeptide of collagen I and III, tissue inhibitor of matrix metalloproteinase 1, carboxyl-terminal telopeptide of collagen type I, and soluble ST2 were measured at baseline (n = 1,135) and 16 (n = 1,113) and 48 weeks (n = 1,016) after randomization. The effects of sacubitril/valsartan on these biomarkers were compared with those of valsartan alone. Baseline biomarker values and changes from baseline to 16 weeks were related to primary endpoint. Results: At baseline, all 5 biomarkers were higher than published referent control values. Sixteen weeks after randomization, sacubitril/valsartan decreased tissue inhibitor of matrix metalloproteinase 1 by 8% (95% confidence interval [CI]: 6% to 10%; p < 0.001), soluble ST2 by 4% (95% CI: 1% to 7%; p = 0.002), and N-terminal propeptide of collagen III by 3% (95% CI: 0% to 6%; p = 0.04) and increased carboxyl-terminal telopeptide of collagen type I by 4% (95% CI: 1% to 8%; p = 0.02) compared with valsartan alone, consistently in men and women and patients with left ventricular ejection fraction above or below the median of 57%. Higher levels of tissue inhibitor of matrix metalloproteinase 1 and soluble ST2 at baseline and increases in these markers at 16 weeks were associated with higher primary endpoint event rates. Conclusions: Biomarkers reflecting extracellular matrix homeostasis are elevated in heart failure with preserved ejection fraction, favorably altered by sacubitril/valsartan, and have important prognostic value. (Prospective Comparison of ARNI With ARB Global Outcomes in HF With Preserved Ejection Fraction [PARAGON-HF]; NCT01920711).