Although Cognitive Assessment System (CAS)-2: Brief was published in 2014 and it is used in applied settings all over the world, we still do not know how well it relates to academic achievement. Thus, this study aimed to examine the relations of CAS-2: Brief with reading and mathematics performance. Two hundred and twenty-two Greek-speaking children in Grade 6 (98 boys, 124 girls, M[subscript age] = 135.85 months) were assessed on CAS-2: Brief and on measures of reading (Wordchains and CBM-Maze) and mathematics (Mathematics Achievement Test and Mathematics Reasoning Test). Results of the correlational analysis showed first that the CAS-2: Brief total score correlated moderately with both reading and mathematics tasks (rs ranged from 0.36 to 0.53). Second, results of the multilevel analyses revealed that the CAS-2: Brief total score explained 11.4% of unique variance in mathematics and 27.6% of unique variance in reading. Theoretical and practical implications of these findings are discussed.