Although recent research suggests that language mindsets (i.e., fundamentalbeliefs aboutthe fixedness and malleability of language learning ability) are important for L2 learners' motivation and learning behaviors, much research has focused on quantitative approaches and static individual differences, with little emphasis on its student-centered and ecologically-relevant phenomena. The aim of this study was to take an ecological perspective to understand the development of mindsets about L2 writingand their relevance tomotivation. Based on an analysis of in-depth interviewswith six (two males, four females) adult EFL learners in Iran, we identified that several eco-systemic factors underlie the emergence, complexity, and dynamics of the learners' mindsets regarding the skill of L2 writing. Students emphasize that teachers (microsystem) play a central role in constructingtheir mindsets about L2 writing. In addition, their previous learning experiences, including teachers, parents, and high-stake exams (mesosystem), the institutional policies that emphasize English oral skills and neglect writing skills (exosystem), and the mainstream culture in favor of a natural talent for writing skills (macrosystem) also contributed to the emergence of learners' mindsets. Moreover, the results highlighted the domain-specific and dynamic nature of language mindsets, such that learners considered their mindsets about L2 writing skills differently from other skills (e.g., speaking) and that their mindsets changed in different stages of the learning processes. We show that the ecological approach can unpack the complex-dynamic and multifaceted nature of mindsets, providing theoretical and pedagogical implications for fostering growth mindsets and improving learners' L2 writing motivation. [ABSTRACT FROM AUTHOR]