The interactions among cognitive, affective, and linguistic factors have received scant attention in task-based writing research. To address this gap, this study aims to examine the relationships among task complexity, task motivation, task engagement, and syntactic complexity in second language (L2) writing. One hundred L2 learners enrolled in an English-as-a-second-language (ESL) writing course at a university in the United States performed simple and complex versions of an argumentative writing task at a 1-week interval. After each task performance, participants completed questionnaires examining their task motivation. Task engagement was measured through time on task and length of production. The essays were analyzed using specific syntactic complexity measures. The results showed that the participants dedicated more time to the complex task and displayed higher motivation levels in two orientations (identified and intention). Furthermore, they produced fewer words when faced with a complex task. Additionally, task complexity influenced only one dimension of syntactic complexity. Finally, time on task predicted two dimensions of syntactic complexity (mean length of T-unit and mean number of complex nominals per clause). These results emphasize the need to consider cognitive, affective, and linguistic factors in task-based writing research. By doing so, educators can develop writing tasks that effectively engage ESL learners, enhance their motivation, and promote their linguistic growth.