Abstarct: Interactive writing is a collaborative and dynamic approach to teaching writing, which encourages enhanced engagement and learning among learners. Writing self-efficacy, the belief in one's ability to effectively accomplish writing tasks, is a main factor in improving writing skills, particularly for learners of English as a Foreign Language (EFL). This study aimed to explore the impact of interactive writing on the writing self-efficacy of intermediate adult EFL learners in Iran. This quasi-experimental study was conducted at a language institute in Sari, Iran, with 60 intermediate EFL learners (29 males, 31 females) divided into control and experimental groups. The participants, aged 20–30 and with at least two years of English learning experience, were selected through convenience sampling. The experimental group received instruction in interactive writing strategies, whereas the control group received traditional instruction. To ensure consistency in language proficiency, the Quick Oxford Placement Test (QOPT) was administered. Additionally, the Writing Self-Efficacy Questionnaire, informed by social cognitive theory, was utilized as both a pretest and posttest measure. The questionnaire assessed five dimensions of writing self-efficacy, namely Linguistic Knowledge Efficacy (LKE), Information Organization Efficacy (IOE), Rehearsal and Memory Efficacy (RME), Self-Regulatory Efficacy (SRE), and Writing Performance Efficacy (WPE). Results of the independent samples t-test revealed no significant difference in pretest scores between the control and experimental groups (p = .492), suggesting comparable baxseline levels of writing self-efficacy. However, posttest results indicated a significant difference between the control group and the experimental group (p = .000), with the experimental group showing a significantly higher mean score. The paired-samples t-test within the experimental group also showed a significant improvement from pretest to posttest (p = .000). The results of this research have pedagogical implications for EFL instruction, stating that incorporating interactive writing activities can lead to greater confidence and proficiency in writing, which ultimately improves overall language learning outcomes.