Current research on interest in English as a Foreign Language (EFL) learning has underscored its role in fostering learner engagement, enhancing motivation, and contributing to academic success. However, little is known about how topic interest affects specific writing quality measures, and even less about how a learner's level of interest in a text influences Google Translate (GT) look-up behavior. Therefore, the main objectives of this study were to assess the impact of topic interest on EFL writing quantity and quality as well as on GT look-up behavior. Sixty Chilean university students completed argumentative writing tasks on both high-interest and low-interest topics under a within-subjects design. The written output was analyzed in terms of writing quantity (i.e., the number of words written), syntactic complexity (measured by T-unit length and the ratio of subordinate clauses to T-units), accuracy, and GT look-up behavior. Results revealed that topic interest significantly affected syntactic complexity measures across conditions but did not significantly impact writing quantity or accuracy. While GT look-up behavior remained consistent across topics, a correlation emerged between GT use and the number of words written in the low-interest condition. The findings suggest that high-interest topics enhance syntactic complexity, whereas low-interest topics may prompt greater reliance on translation tools. The study highlights the role of topic interest in writing development and discusses pedagogical implications for supporting EFL learners’ writing outcomes.