Study Objectives: Lost productivity caused by insomnia is a common and costly problem for employers. Although evidence for the efficacy of Internet-based cognitive behavioral therapy for insomnia (iCBT-I) already exists, little is known about its economic effects. This study aims to evaluate the cost-effectiveness and cost-benefit of providing iCBT-I to symptomatic employees from the employer's perspective. Methods: School teachers (N = 128) with clinically significant insomnia symptoms and work-related rumination were randomized to guided iCBT-I or a waitlist-control-group, both with access to treatment as usual. Economic data were collected at baseline and 6-mo follow-up. We conducted (1) a costeffectiveness analysis with treatment response (Reliable Change [decline of 5.01 points] and Insomnia Severity Index < 8 at 6-month follow-up) as the outcome and (2) a cost-benefit analysis. Because both analyses were performed from the employer's perspective, we focused specifically on absenteeism and presenteeism costs. Statistical uncertainty was estimated using bootstrapping. Results: Assuming intervention costs of €200 ($245), cost-effectiveness analyses showed that at a willingness-to-pay of €0 for each positive treatment response, there is an 87% probability that the intervention is more cost effective than treatment as usual alone. Cost-benefit analyses led to a net benefit of €418 (95% confidence interval: -593.03 to 1,488.70) ($512) per participant and a return on investment of 208% (95% confidence interval: -296.52 to 744.35). The reduction in costs was mainly driven by the effects of the intervention on presenteeism and to a lesser degree by reduced absenteeism. Conclusions: Focusing on sleep improvement using iCBT-I may be a cost-effective strategy in occupational health care.