--- - |2 Algebraic competence is a major determinant of college access and career prospects, and equal sign knowledge is taken to be foundational to algebra knowledge. However, few studies have documented a causal effect of early equal sign knowledge on later algebra skill. This study assessed whether second‐grade students’ equal sign knowledge prospectively predicts their fourth‐grade algebra knowledge, when controlling for demographic and individual difference factors. Children (N = 177; Mage = 7.61) were assessed on a battery of tests in Grade 2 and on algebraic knowledge in Grade 4. Second‐grade equal sign knowledge was a powerful predictor of these algebraic skills. Findings are discussed in terms of the importance of foregrounding equal sign knowledge to promote effective pedagogy and educational equity. - Child Development, EarlyView.