Significance Tie1 and Tie2 receptor tyrosine kinases are key regulators of blood and lymphatic vessel development and of pathological processes including tumor angiogenesis, atherosclerosis, and vascular leakage, e.g., in sepsis. Tie1 is essential for the Tie2 agonist activity of angiopoietins, and the activated receptors form heteromeric complexes in endothelial cell–cell junctions. However, little is known about the activation mechanisms of the Tie receptors. Here we demonstrate that the membrane-proximal domains of Tie2 mediate homotypic interactions, which occur via intermolecular β-sheet formation and are necessary for Tie2 activation. The structural analysis suggests that Tie2/Tie1 heterodimerization occurs by the same mechanism. The crystal structures provide a model for angiopoietin-stimulated Tie2 ectodomain dimerization, clustering, and activation and insights into therapeutic targeting.