OBJECTIVE Comorbid depression is common in patients with type 1 and type 2 diabetes, adversely affecting quality of life, diabetes outcomes, and mortality. Depression can be effectively treated with cognitive behavior therapy (CBT). The Internet is a new and attractive method for delivering CBT intervention on a large scale at relatively low costs. This study evaluated the effectiveness of Web-based CBT for depression treatment in adults with type 1 or type 2 diabetes, with minimal guidance. RESEARCH DESIGN AND METHODS A randomized controlled trial was conducted in the Netherlands in 255 adult diabetic patients with elevated depressive symptoms. Primary outcomes were depressive symptoms. Secondary outcomes were diabetes-specific emotional distress and glycemic control. Assessments were at baseline, after treatment, and at the 1-month follow-up. RESULTS The Web-based CBT was effective in reducing depressive symptoms by intention-to-treat analyses (P = 0.04, d = 0.29; clinical improvement 41% vs. 24% P < 0.001) and by per-protocol analyses (P < 0.001, d = 0.70; clinical improvement, 56% vs. 24% P < 0.001). The intervention reduced diabetes-specific emotional distress (P = 0.03) but had no beneficial effect on glycemic control (P > 0.05). CONCLUSIONS Web-based CBT depression treatment is effective in reducing depressive symptoms in adults with type 1 and type 2 diabetes. In addition, the intervention reduces diabetes-specific emotional distress in depressed patients.