Surface-stress changes induced by the adsorption of oxygen and hydrogen on clean Pt(111) are measured at 320 K under UHV conditions with an optical crystal curvature method and are calculated using density-functional theory. A compressive surface-stress change is measured upon exposure of Pt(111) to either H₂ or O₂. The magnitude of the adsorbate-induced surface-stress change is in quantitative agreement with our calculations. Our results indicate that both electropositive (H) and electronegative (O) adsorbates may induce a compressive surface-stress change. The significance of these results for the understanding of adsorbate-induced surface stress is discussed.