Aim: The study aimed to critically review and synthesize the best available evidence about the effectiveness of therapist-guided internet-delivered cognitive behavioral therapy (iCBT) in terms of reducing sickness absence (SA). Methods: We searched Medline (PubMed), Embase, PsycInfo, CINAHL, and Cochrane Central (up to November 2020) for English language peer-reviewed papers that described randomized controlled trials of therapist-guided iCBT compared with usual treatment for SA in adults with common mental disorders. Eligible studies were assessed with the Cochrane Risk of Bias 1 tool, meta-analysis was conducted using a random-effects model, and standardized mean differences (SMD) with 95% confidence intervals (CI) were reported. A subgroup analysis investigated potential moderating variables (diagnosis, SA at baseline, and estimated accuracy of self-report). Results: We identified 2788 references, of which 68 remained after the completion of the systematic screening process. A hand search of reference lists yielded no additional studies. The full texts of these 68 studies were appraised critically, and 11 were deemed to be suitable for a meta-analysis. SA was similar for iCBT and usual treatment groups (SMD: 0.02, 95% CI, -0.08 to 0.11), and remained similar even after the removal of two studies in which the recall time was over 3 months (SMD: 0.00, -0.11 to 0.12). Similar SA levels in intervention and control groups at 6-month and 12-month follow-up were observed in studies of participants with depression symptoms. Conclusions: iCBT did not appear to be effective in terms of reducing (largely self-assessed) SA in adults with common mental disorders. There is a need to improve the method and consistency of assessing SA.