["TESOL Quarterly, EarlyView. ", "\nAbstract\nWriting is crucial in tertiary education, yet enhancing the complexity of academic writing presents significant challenges for second language (L2) learners. This study explores the potential of dialogue journal writing (DJW), an interactive and low‐stress classroom activity, to enhance writing complexity among novice L2 writers. Employing a mixed‐methods research (MMR) design, we investigated the effects of DJW and learners' motivational beliefs on the syntactic and lexical complexity of L2 writings over one academic year. Forty‐nine first‐year undergraduates from a Taiwanese technical university participated, producing nine general writings in the first semester and seven academic writings in the second. Learners' motivational beliefs, including self‐efficacy and interest, were elicited from reflections and quantized as categorical variables for analysis using a reflective coding‐based approach. Mixed effects modeling assessed changes in syntactic and lexical complexity and the impact of motivational beliefs. Results indicated that DJW significantly enhanced syntactic complexity in both general and academic writing, but lexical complexity improved only in general writing. The non‐significant findings regarding motivational beliefs suggest a potentially nuanced relationship with writing complexity. This study demonstrates how fluency‐building interactive writing practices can bridge the gap between novice and intermediate L2 writers and contributes to the application of MMR in L2 writing research.\n"]