Teaching English remains a pivotal issue in education, particularly in Iraq, where specialists and researchers strive to enhance teaching methodologies and curricula. This study aims to improve Iraqi secondary school students' writing performance through the application of the genre-based approach while examining the influence of interlanguage on their target language acquisition. The research involved 46 second-year secondary school students divided into an experimental group and a control group, each consisting of 26 participants. Data collection methods included pre-and post-questionnaires, pre-and post-tests, observations, case studies, and focus group discussions. Quantitative data were analyzed using Excel and SPSS, while qualitative data were examined through thematic analysis. Results indicated significant improvement in the experimental group's writing, with reduced grammatical, spelling, and punctuation errors. Furthermore, interlanguage interference was identified as a substantial factor affecting students' writing.