Abstract Purpose Calcium dobesilate (CaD) is used in the treatment of diabetic retinopathy, but its mechanism of action is not completely elucidated. We investigated the molecular and cellular mechanisms underlying the protective effects of CaD against the blood‐retinal barrier (BRB) breakdown induced by diabetes. Methods Wistar rats were divided into: controls, diabetic (1 month diabetes duration) and diabetic treated with CaD (100 mg/kg/day; orally given) during the last 10 days. The BRB breakdown was evaluated using Evans blue. The content or distribution of occludin, claudin‐5, ZO‐1, ICAM‐1 and p38 MAPK was evaluated by western blotting and immunohistochemistry. Leukocyte adhesion was evaluated in retinal vessels and in vitro. Oxidative stress was evaluated by the detection of oxidized carbonyls and tyrosine nitration. NF‐κB activation was measured by ELISA. Results Diabetes increased the BRB permeability and retinal thickness, decreased occludin and claudin‐5 levels, and altered the distribution of ZO‐1 and occludin in retinal vessels. CaD inhibited these changes, as well as the increase in ICAM‐1 levels and leukocyte adhesion to retinal vessels or endothelial cells induced by diabetes or high glucose. Moreover, CaD decreased oxidative stress, and p38 MAPK and NF‐κB activation caused by diabetes. Conclusion CaD prevents the BRB breakdown induced by diabetes by restoring tight junction protein levels and organization and decreasing leukocyte adhesion to retinal vessels. The protective effects of CaD are likely to involve the inhibition of p38 MAPK and NF‐κB activation, possibly through the inhibition of oxidative/nitrosative stress. Support: OM Pharma SA, Switzerland