The integration of artificial intelligence (AI)-assisted writing tools in English language teaching has gained increasing attention due to the potential to support students’ writing development. Grammarly, as an AI-powered writing assistant, provides feedback on grammar, spelling, punctuation, and writing accuracy, making it relevant to writing instruction. However, studies examining the integration of Grammarly within a Process-Based Writing Approach (PBWA), particularly in junior high school contexts, remain limited. This study aimed to investigate the implementation of a Grammarly-mediated PBWA in teaching descriptive writing and to explore the challenges experienced by students during its implementation. Employing a qualitative case study design, the study was conducted at a junior high school in Karawang. The participants consisted of one English teacher and ten seventh-grade students. All ten students participated in the classroom observations and document analysis of their writing drafts, while five students were purposively selected for semi-structured interviews. Data were collected through classroom observations, semi-structured interviews, and document analysis and were analyzed using thematic analysis. The findings revealed that the implementation involved four stages: planning, drafting, revising, and editing, with Grammarly mainly supporting the revising and editing stages. Several challenges were identified, including technological limitations, affective challenges, linguistic difficulties, and over-reliance on Grammarly suggestions. The study concludes that Grammarly can effectively support PBWA when accompanied by appropriate teacher guidance.