OBJECTIVE Depression is common in patients with type 2 diabetes and adversely affects quality of life and diabetes outcomes. We assessed whether light therapy, an antidepressant, improves mood and insulin sensitivity in patients with depression and type 2 diabetes. RESEARCH DESIGN AND METHODS This randomized, double-blind, placebo-controlled trial included 83 patients with depression and type 2 diabetes. The intervention comprised 4 weeks of light therapy (10,000 lux) or placebo light therapy daily at home. Primary outcomes included depressive symptoms (Inventory of Depressive Symptomatology [IDS]) and insulin sensitivity (M-value derived from the results of a hyperinsulinemic-euglycemic clamp). Secondary outcomes were related psychological and glucometabolic measures. RESULTS Intention-to-treat analysis showed that light therapy was not superior to placebo in reducing depressive symptoms (23.9 IDS points [95% CI 29.0 to 1.2]; P = 0.248) and had no effect on insulin sensitivity (0.15 mg/kg*min [95% CI 20.41 to 0.70]; P = 0.608). Analyses incorporating only those participants who accurately adhered to the light therapy protocol (n = 51) provided similar results, but did suggest positive effects of light therapy on depression response rates (‡50% reduction in IDS points) (26% more response; P = 0.031). Prespecified analysis showed effect moderation by baseline insulin sensitivity (P = 0.009) and use of glucose-lowering medication (P = 0.023). Light therapy did not affect depressive symptoms in participants with higher insulin sensitivity or those who use only oral glucose-lowering medication or none at all, but it did produce a relevant effect in participants with lower insulin sensitivity (212.9 IDS points [95% CI 221.6 to 24.2]; P = 0.017) and a trend toward effectiveness in those using insulin (212.2 IDS points [95% CI 221.3 to 23.1]; P = 0.094). Light therapy was well tolerated. CONCLUSIONS Although this trial is essentially inconclusive, secondary analyses indicate that light therapy might be a promising treatment for depression among a subgroup of highly insulin-resistant individuals with type 2 diabetes.