While previous studies on online resource use in L2 writing have focused on the overall writing quality, limited attention has been paid to its effects on linguistic complexity and real-time writing processes. Addressing this gap, the present study explored how online resource use influences both the processes and products of L2 writing. Forty-nine intermediate L2 learners completed two computer-mediated argumentative writing tasks, either with or without the use of online resources. Writing behaviors were captured via keystroke logging and screen recording, and analyzed for search activity, fluency, pausing, and revision quantity. Cognitive processes were examined through stimulated recall interviews, and written products were evaluated for both quality and linguistic complexity. The results showed that participants spent an average of 14 % of task time using online resources, with considerable individual variation. Mixed-effects modeling revealed that resource use facilitated the production of more sophisticated words, with marginal influence on writing quality or syntactic complexity. Resource use was also associated with longer between-word pauses, fewer within-word pauses, and reduced revisions. These findings highlight the potential of online resource use to enhance the authenticity of L2 writing assessment tasks without compromising test validity, while encouraging the use of more advanced vocabulary in writing. • Learners spent 14 % of the total writing task time using online resources. • Online resource use had no significant impact on L2 writing quality. • Online resource use improved lexical sophistication, not syntactic complexity. • Online resource use reduced within-word pauses and aided spelling retrieval. • Online resource use led to fewer revisions but did not affect fluency.