To reach sites of inflammation, neutrophils execute a series of adhesion and migration events that include transmigration through the vascular endothelium and chemotaxis through the vicinal extracellular matrix until contact is made with the point of injury or infection. These in vivo microenvironments differ in their mechanical properties. Using polyacrylamide gels of physiologically relevant elasticity in the range of 5 to 100 kPa and coated with fibronectin, we tested how neutrophil adhesion, spreading, and migration were affected by substrate stiffness. Neutrophils on the softest gels showed only small changes in spread area, whereas on the stiffest gels they showed a 3-fold increase. During adhesion and migration, the magnitudes of the distortions induced in the gel substrate were independent of substrate stiffness, corresponding to the generation of significantly larger traction stresses on the stiffer gels. Cells migrated more slowly but more persistently on stiffer substrates, which resulted in neutrophils moving greater distances over time despite their slower speeds. The largest tractions were localized to the posterior of migrating neutrophils and were independent of substrate stiffness. Finally, the phosphatidylinositol 3-kinase inhibitor LY294002 obviated the ability to sense substrate stiffness, suggesting that phosphatidylinositol 3-kinase plays a mechanistic role in neutrophil mechanosensing.