This study explored the effects of explicit instruction in metadiscourse markers on the business letter-writing proficiency of ESP students majoring in Business Administration at Payame Noor University-Rey Branch. Given the crucial role of metadiscourse markers in fostering coherence and engaging readers, the research sought to evaluate whether focused instruction could improve learners’ writing performance. A total of 60 ESP students were homogenized and randomly assigned to an experimental group (n = 30) and a control group (n = 30). The experimental group underwent explicit task-based instruction emphasizing the use of metadiscourse markers in business letter-writing tasks, while the control group followed a conventional instructional approach without such emphasis. Both groups participated in pre-test and post-test evaluations to measure changes in writing quality. An ANCOVA analysis revealed a significant improvement in the writing performance of the experimental group compared to the control group, F(1, 57) = 14.86, p = .001, p < .01. The findings underscore the pedagogical value of integrating metadiscourse marker instruction into ESP writing courses, highlighting its potential to enhance clarity, persuasiveness, and overall quality in students’ business letters. These results offer actionable insights for language educators and curriculum developers seeking to improve writing instruction in ESP contexts.