Endothelial cell (EC) sensing of fluid shear stress direction is a critical determinant of vascular health and disease. Unidirectional flow induces EC alignment and vascular homeostasis, whereas bidirectional flow has pathophysiological effects. EC express several mechanoreceptors that respond to flow but the mechanism for sensing shear stress direction is poorly understood. We observed using in vitro flow systems and magnetic tweezers that β1 integrin is a key sensor of force direction because it is activated by unidirectional but not bidirectional shearing forces. β1 integrin activation by unidirectional force was amplified in EC that were pre-sheared in the same direction, indicating that alignment and β1 integrin activity has a feedforward interaction which is a hallmark of system stability. En face staining and EC-specific genetic deletion studies of the murine aorta revealed that β1 integrin is activated and is essential for EC alignment at sites of unidirectional flow but is not activated at sites of bidirectional flow. In summary, β1 integrin sensing of unidirectional force is a key mechanism for decoding blood flow mechanics to promote vascular homeostasis.