Some recent studies have reported positive effects of artificial intelligence (AI)-powered writing assistants on students' EFL writing skills, but their impact on affective factors has yet to be examined. Thus, the present study investigated the effects of a QuillBot-based intervention on English Language majors' EFL writing performance, apprehension, and self-efficacy. The participants constituted 18 fourth-year students in the English Language Department, Faculty of Archaeology and Languages, Matrouh University, during the second semester of the 2023-2024 academic year. For six weeks, students performed in-class and out-of-class QuillBot-based activities where they collaboratively developed their first drafts, reviewed the AI-generated feedback, revised their writing, and wrote their final drafts. One test and two scales were administered before and after experimentation to measure the effects of the proposed intervention. Qualitative data was also collected from the students' reflective reports to gain deeper insights into their perceptions of the intervention in improving their writing and affective states. The results revealed that the QuillBot-based intervention had significant positive effects on students' writing performance, apprehension, and self-efficacy. The students also displayed largely positive perceptions toward it. Such results suggest that using QuillBot collaboratively can enhance writing performance, reduce writing apprehension, and promote writing self-efficacy among EFL learners.