Motivational regulation has long been recognized as an essential but insufficiently investigated aspect of self-regulated learning (SRL), especially in relation to learning English-as-a-foreign-language (EFL) writing. This study intends to fill the gap by investigating the predictive effect of motivational regulation strategies on EFL students' writing performance mediated by SRL strategies. Data were collected from undergraduate students in mainland China (N = 512) through self-report questionnaires and an English writing test. Results of structural equation modeling (SEM) confirmed a partial mediation model in which motivational regulation strategies, as a whole, not only had direct and indirect effects on students' writing performance but were also significantly correlated with their reported use of SRL strategies relating to cognition, metacognition, and social behavior. In addition, only cognitive and metacognitive strategies were found to be significant mediators in the model while social behavior strategies were not. The findings suggest that cumulative knowledge of motivational regulation is an antecedent of the reported use of other SRL strategies in affecting EFL writing performance. The inclusion of SRL strategies in the mediation model also contributes to a clear understanding of L2 writing processes within the SRL mechanism for improving writing outcomes.