The current study investigates the effectiveness of grade-ten students' reflective science journal writing on their self-regulated learning strategies. We used a pre-post control group quasi-experimental design. The sample consisted of 62 tenth-grade students (15 years old) in Oman, comprising 32 students in the experimental group and 30 students in the control group. Both groups studied a science text unit called 'Matter and Energy in Chemical Reactions'. Students in the experimental group were given a model for a journal, which they wrote after they finished their science lessons. They reflected on their dialogues with their teacher and classmates. They also reflected on their scientific observations, their main conclusions, their evaluation of their level of understanding of the scientific concepts presented in the lesson, their achievement of the lesson goals, and their personal feelings regarding what was taught in the lesson. The control group studied the same unit without writing reflective journals. We used a modified self-regulation strategy instrument to measure the effectiveness of treatment. The results showed that participants in the journal-writing group (experimental group) (M = 3.96; SD = 0.37) significantly outperformed participants in the control group (M = 3.62; SD = 0.28) with respect to their self-regulation strategies. The study recommends that reflective journal-writing should be encouraged by science teachers and in science textbooks.