Background It remains unclear whether the novel biomarker cysteine-rich angiogenic inducer 61 (CCN1) adds incremental prognostic value to the GRACE 2.0 (Global Registry of Acute Coronary Events) risk score and biomarkers high-sensitivity Troponin T, hsCRP (high-sensitivity C-reactive protein), and NT-proBNP (N-terminal pro-B-type natriuretic peptide) in patients with acute coronary syndromes. Methods and Results Patients referred for coronary angiography with a primary diagnosis of acute coronary syndromes were enrolled in the Special Program University Medicine - Acute Coronary Syndromes and Inflammation cohort. The primary/secondary end points were 30-day/1-year all-cause mortality and the composite of all-cause mortality or myocardial infarction as used in the GRACE risk score. Associations between biomarkers and outcome were assessed using log-transformed biomarker values and the GRACE risk score (versions 1.0 and 2.0). The incremental value of CCN1 beyond a reference model was assessed using Harrell's C-statistics calculated from a Cox proportional-hazard model. The P value of the C-statistics was derived from a likelihood ratio test. Among 2168 patients recruited, 1732 could be analyzed. CCN1 was the strongest single predictor of all-cause mortality at 30 days (hazard ratio [HR], 1.77 [1.31, 2.40]) and 1 year (HR, 1.81 [1.47, 2.22]). Adding CCN1 alone to the GRACE 2.0 risk score improved C-statistics for prognostic accuracy of all-cause mortality at 30 days (0.87-0.88) and 1 year (0.81-0.82) and when combined with high-sensitivity Troponin T, hsCRP, NT-proBNP for 30 days (0.87-0.91), and for 1-year follow-up (0.81-0.84). CCN1 also increased the prognostic value for the composite of all-cause mortality or myocardial infarction. Conclusions CCN1 predicts adverse outcomes in patients with acute coronary syndromes adding incremental information to the GRACE risk score, suggesting distinct underlying molecular mechanisms. Registration URL: https://www.clinicaltrials.gov. Unique identifier: NCT01000701.