This study explores the effect of mobile-assisted task-based learning (M-TBL) on vocabulary achievement and student attitude among English as a in content and language integrated learning (CLIL) science lessons, the researcher focused on foreign language (FL) students. To investigate this, a quasi-experimental research design was utilized. In the first semester of the 2022-2023 academic year, 34 fourth-grade students from a private school in Istanbul took part in the study. During the course of 6 weeks, the experimental group engaged in six different M-TBL activities and were assigned a one-page worksheet as homework, while the control group completed only the one-page worksheet. Quantitative data for the study were collected through vocabulary achievement tests and a student attitude questionnaire. The data underwent analysis through paired sample t-tests and descriptive analyses. The results indicated a significant difference in vocabulary achievement between the experimental and control groups, and they also revealed a positive impact on student attitudes, igniting their interest in learning the target language. In light of these findings, it can be concluded that mobile-assisted task-based activities yield positive effects on both vocabulary achievement and student attitude in primary FL learners participating in CLIL science lessons.