Background: Teaching is often characterized as a stressful profession, with a substantial proportion of teachers experiencing chronic stress and burnout. Research indicates that teachers often experience stress upon entering the workforce, leading to detrimental effects on their health, occupational well-being, and work performance and adversely impacting student outcomes. While meta-analyses have demonstrated the efficacy of internet-based stress management interventions (iSMIs) for both experienced professionals and university students, there remains a gap in research on the efficacy of iSMIs tailored to teachers and career starters. Objective: We tailored an iSMI to beginning teachers and added a newly developed web-based classroom management skill training. This study examined the effectiveness of the tailored iSMI in reducing perceived stress and improving further secondary outcomes. In addition, this study was the first to explore problem-solving ability and emotion regulation skills as potential mechanisms of change within an iSMI grounded in transactional stress theory. Methods: Participants were 200 highly stressed beginning teachers (Perceived Stress Scale score of >21) undergoing German teacher induction randomized to either an intervention group (IG) with guidance or a waitlist control group. Outcomes were assessed online at baseline, 8 weeks after randomization (postintervention time point; T2), and at both the 3-month (T3) and extended 6-month follow-up. At T2, data were collected from 84% (84/100) of the participants in the IG and 90% (90/100) of the participants in the waitlist control group. Results: The IG reported significant, practically meaningful, and sustained improvements in both perceived stress (T2: d=0.52, 95% CI 0.24-0.80, and P<.001; T3: d=0.49, 95% CI 0.21-0.77, and P<.001) and most secondary outcomes in the intention-to-treat analysis using analyses of covariance. Postintervention (T2) effects on mental health indicators, including depression, work-related rumination, anxiety, and insomnia, were substantial (d≥0.60), whereas no significant effects were observed for absenteeism (P=.22) and presenteeism (P=.80). The investigated mediators, problem-solving ability (d=0.57) and emotion regulation skills (d=0.69), improved. Moreover, parallel mediation analysis revealed that the iSMI exerted its effect on perceived stress through improved problem-solving ability (indirect path a₂b₂=–0.77, 95% CI –1.50 to –0.04) and emotion regulation skills (indirect path a₁b₁=–0.97, 95% CI –1.73 to –0.22). Conclusions: This study contributes to the growing body of evidence on iSMIs for beginning teachers during a highly demanding transition phase and supports the integration of these interventions into teacher training programs. Findings from the mediation analysis suggest that both problem-focused and emotion-focused coping strategies should be equally considered in stress management interventions. This strengthens the program theory based on the transactional stress model. Finally, these findings encourage further research on iSMIs for other groups of career starters to support their transition into the workforce.