["TESOL Quarterly, EarlyView. ", "\nAbstract\nThis study examined how task design, task motivation, and learner engagement influenced L2 vocabulary learning by comparing learner‐generated content (LGC) and teacher‐generated content (TGC) in a classroom setting. One hundred sixty‐three Japanese high school students from five intact classes were assigned to LGC, TGC, or a test‐only control group, and the two treatment groups completed two decision‐making tasks. Vocabulary learning was assessed using form recall, meaning recall, and a lexicosemantic judgment task (LJT) with a pretest–posttest design. Task motivation (expectancies, value, and cost) was measured prior to Task 1, and learner engagement (cognitive, social, and emotional) was measured after Task 2. Generalized linear mixed‐effects models showed that the LGC group outperformed the TGC group on form recall (large effect) and meaning recall (medium effect), whereas no group difference emerged for LJT performance. Engagement affected vocabulary learning: social engagement was negatively associated with meaning‐recall accuracy. Although task motivation showed limited direct associations with vocabulary learning, it was a significant predictor of engagement, suggesting the indirect influence of motivation on learning via engagement. This study provides classroom‐based evidence that a learner‐centered approach can support vocabulary learning and offers insights into the nuanced relationship among task motivation, engagement, and vocabulary acquisition.\n"]