Autonomy as an essential factor contributing to successful second language learning has been extensively studied in recent decades. Despite the importance of autonomy, the casual effect of the negotiated syllabus on enhancing learner autonomy through empirical studies is understudied. Few studies have attempted to document ESP learners’ experience with the negotiated syllabus. Therefore, this study methodologically contributes to the body of knowledge in autonomy by addressing the gaps mentioned above. This mix-methods research used a quasi-experimental design in the quantitative phase to investigate the effects of the negotiated syllabus on Iranian ESP learners’ (N = 71) autonomy. The experimental treatment was carried out with 32 students in the experimental class over eight weeks. The treatment comprised class-time exposure to a negotiated syllabus co-constructed through negotiation of purposes, contents, methodsو and evaluation. The control group used a pre-designed syllabus. T-test analysis showed a significant increase in the autonomy level of the experimental class in comparison to their counterparts in the control group. Qualitative data collected through semi-structured interview and student journal also confirmed the results. The compelling evidence on the positive effect of negotiated syllabus has different implications for different stakeholders including policymakers and teacher trainers. پرونده مقاله