Significance The human circulatory system includes the heart, arteries, veins, and capillaries, all of which are lined by endothelial cells to prevent the escape of vascular contents into surrounding tissues. Platelets are blood cells that are best known for their essential role in preventing blood loss after injury. Here, we employed sensitive methods to measure changes in vascular barrier function in vivo and used them to identify a role for platelets in maintaining vascular homeostasis. The results highlight the role of platelet collagen receptors and dense granules, show which vessels are particularly dependent on platelets for this purpose, and point to an underappreciated interchange between platelets and endothelial cells that is lost in severe thrombocytopenia.