Objective: This study reported a randomized controlled trial of the effectiveness of an Internet-based cognitive behavioral therapy (iCBT) program on work engagement and secondary work-related outcomes. Methods: Participants who fulfilled the inclusion criteria were randomly allocated to an intervention or a control group (N = 381 for each). A 6-week, 6-lesson iCBT program using a Manga (Japanese comic) story was provided only to the intervention group.Work engagement was assessed at baseline and at 3- and 6-month follow-ups for both groups. Results: The iCBT program showed a significant intervention effect on work engagement (P = 0.04) with small effect sizes (Cohen's d = 0.16 at 6-month follow-up). Conclusions: The study showed computerized cognitive behavior therapy delivered via the Internet to be effective (with a small effect size) in increasing work engagement in the general working population. Trial Registration: UMIN Clinical Trials Registry (UMIN-CTR) UMIN000006210.