BACKGROUND: Severe fatigue is a common and distressing symptom affecting approximately one in four survivors of breast cancer. The current study examined the efficacy of Internet-based cognitive behavioral therapy (ICBT) for severe fatigue in survivors of breast cancer compared with care as usual (CAU). METHODS: The authors conducted a parallel-group randomized controlled trial. Severely fatigued, disease-free survivors of breast cancer who had completed cancer treatment at least 3 months previously were eligible. Participants were randomly allocated to ICBT or CAU using computer-generated stratified block randomization. The primary outcome of fatigue severity was assessed at baseline and after 6 months, as were the secondary outcomes of functional impairment, psychological distress, and quality of life. Statistical effects were tested with analyses of covariance (intention-to-treat analysis). RESULTS: Participants were recruited between January 2014 and March 2016 and assigned to ICBT (66 patients) or CAU (66 patients). Compared with the participants who had received CAU, those who had received ICBT reported lower fatigue scores at 6 months (mean difference [Δ], 11.5; 95% confidence interval [95% CI], 7.7-15.3) and a large effect size (Cohen d = 1.0), with the majority of patients (73%) demonstrating clinically significant improvement. ICBT also was found to lead to lower functional impairment (Δ, 297.8; 95% CI, 145.5-450.1) and psychological distress scores (Δ, 5.7; 95% CI, 3.4-7.9) and higher quality-of-life scores (Δ, 11.7; 95% CI, 5.8-17.7) compared with CAU, with medium to large effect sizes (Cohen d = 0.6-0.8). CONCLUSIONS: ICBT appears to be effective in reducing severe fatigue and related symptoms and meets the current need for easy accessible and more efficient evidence-based treatment options for severely fatigued survivors of breast cancer. Cancer 2017;123:3825-34.