Background and Purpose: Foreign language learning has long been a topic of research, with pedagogical challenges frequently examined. While self-regulation is recognized as critical to student motivation and success, the effectiveness of specific self-regulatory strategies in foreign language learning merits further investigation. This study sought to explore the impact of a self-regulatory strategy, mental contrasting with implementation intentions (MCII), on the foreign language learning performance of secondary students. Methodology: This quasi-experimental study involved 47 fifth-grade students attending a foreign language course at a secondary school. Participants were assigned to an MCII group (n=27) or a comparison group (n=20). The MCII group received instruction on identifying goals, envisioning the best outcome, anticipating obstacles, and creating plans to overcome them. The comparison group received instruction on identifying goals and envisioning the best outcome. Grade point averages in the English course from the first and second semesters were used as pre-test and post-test measures. A general linear model was used to analyse the data. Findings: MCII group performed significantly better in the foreign language course at the end of the semester than the comparison group, with a medium effect size (η2p = .11). Participants in the MCII condition achieved higher GPA difference scores (M= 8.15, SD = 10.05) than those in the comparison condition (M= 2.82, SD= 6.17). Contributions: MCII strategy fosters goal-directed behaviour, goal striving, and ultimately, goal achievement. Practitioners can create and use MCII forms tailored to students' grade levels to enhance their goal striving and achievement in foreign language learning. Keywords: Foreign language learning, intervention, self-regulation, academic performance, quasi-experimental study. Cite as: Caner-Yıldırım, S. (2024). Boosting language learning with mcii strategy: A quasi-experimental study on secondary students. Journal of Nusantara Studies, 9(2), 218-240. http://dx.doi.org/10.24200/jonus.vol9iss2pp218-240