Using Gyeonggi Educational Panel Study data on 4051 Korean students (N = 4051; 1967 female, 2084 male) from 63 middle schools, this longitudinal study investigated the growth trajectory for second/foreign language (L2) achievement and associations with various L2 motivational factors by using a multilevel latent growth curve model in M"plus" 8.4. Because of the hierarchal data structure, the predictive factors were sorted into three student-level subcategories: individual characteristics; L2 classroom engagement components; and family background. At the school level, school SES and teacher efficacy were specified. Results indicate that global and L2 self-efficacy affected the high initial scores; however, no effect was observed on the growth rate. Unexpectedly, L2 interest was found to have no effects, which suggested a possible engagement mediation effect. Finally, of the L2 classroom engagement factors, L2 class comprehension was found to have a strong explanatory power for the initial L2 achievement scores, but exhibited no impact on the growth rate. These findings have second language acquisition implications for practice and interventions.