This qualitative study was conducted in a private school in Abu Dhabi, the United Arab Emirates, to explore the role of reflective diaries as a form of formative assessment in promoting self-regulated learning (SRL) among English as a Second Language learners. Specifically, the study aimed to discover the impact of reflective diaries on students' cognition and metacognition as well as the role they play in increasing students' motivation. Data were collected via semi-structured interviews with six students from grade six and reflective diaries from twenty students. Zimmerman's cyclic model of self-regulated learning and Vygotsky's Social Constructive theory were used as frameworks for understanding the role of reflective diaries in promoting self-regulated learning. Using a thematic analysis, two categories of themes emerged as research questions one: what is the effect of a reflective diary on students' cognition/epistemological beliefs? (comprehension of knowledge and utilization of knowledge); and two categories of themes emerged as research question two: what is the effect of a reflective diary on students' metacognition? (awareness of knowledge and regulation of cognition); one theme emerged as research question three: what is the role of reflective diaries in increasing students' motivation? (attending to feelings). The study identifies its limitations and recommendations for future research.