We derived an index of high school academic rigor (HSAR) by optimizing the prediction of first-year college GPA (FYGPA) based on high school courses taken, grades, and indicators of advanced coursework. Using a large data set and nominal parameterization of high school course outcomes, the HSAR index capitalizes on differential contributions across courses and nonlinear relationships between course grades and FYGPA. Test scores from eighth grade were incorporated in the model to isolate the contribution of HSAR. High school courses with the largest relationships with FYGPA were English 11, English 12, Chemistry, English 10, Calculus, and Algebra 2. Participation in Advanced Placement, accelerated, or honors courses increased HSAR. The correlation of the HSAR index and FYGPA was 0.52 and the HSAR index led to modest improvement in overall prediction when combined with high school GPA and ACT Composite score. HSAR index subgroup differences were smaller than subgroup differences in ACT Composite score. Implications for high school counselors, researchers, and postsecondary student service personnel are discussed.