Understanding of the complement principle has been proposed as closely related to computational skills, but few studies have investigated their interrelations. The present longitudinal study attempted to clarify the picture by examining their potential cross-lagged relation. Fourth graders (n = 221) in Hong Kong received 3 cognitive assessments at intervals of 6 months, consisting of multiple measures of complement understanding, a nonverbal intelligence test, and a computational skills measure. A random intercept cross-lagged panel model revealed that complement understanding significantly predicted future computational skills, but the reciprocal prediction was nonsignificant. The findings provide empirical evidence supporting the close relation between complement understanding and computational skills and call for future endeavors to examine the interrelations between computational skills and understanding of other arithmetic principles. (PsycInfo Database Record (c) 2023 APA, all rights reserved).