Despite the significance of grit and motivational regulation strategies (MRS) to language learning, limited research has been conducted on their longitudinal interplay. The present study explores the relationship between these two constructs in an English as a second language (L2) learning context through a longitudinal design. This study utilizes repeated measures of L2 grit and MRS at two time points (T1 and T2) to investigate the direction of influence between them and proposes and evaluates four models: an autoregressive model, two unidirectional models, and a bidirectional model. Using questionnaire data from 205 Chinese university students to evaluate the models, results from model assessment revealed that L2 grit at T1 positively predicted L2 grit at T2 and that MRS at T1 positively predicted MRS at T2. While L2 grit showed stability, MRS developed over time. The unidirectional model with L2 grit at T1 as a predictor of MRS at T2 was identified as the model of best fit, indicating that L2 grit at an earlier time unidirectionally influenced MRS at a later time. Based on these findings, we conclude that there is a unidirectional influence between L2 grit and MRS: the former plays an influential role in shaping MRS in L2 learning over time. Implications and recommendations for future research are discussed. This study contributes to a deeper understanding of the dynamics between the two constructs, which in turn impacts language learning persistence and success, offering valuable insights for educators, policymakers, and researchers striving to optimize language learning environments and interventions.