AbstractLocal variations in the stiffness at surfaces may affect the elastic response of nanostructures, yet experiments disagree on the magnitude and even sign of the surface excess elastic constants. The present study reports the variation in the effective macroscopic stiffness of bulk samples of nanoporous gold when the surface state is modulated under potential control in an electrochemical environment. Using in situ experiments in a dynamic mechanical analyzer to measure the storage and loss moduli, we show that adsorption of ⩽1⩽1 atomic monolayer of oxygen species as well as a capacitively controlled excess of electrons at the surface stiffen the material, while oxygen desorption/electron depletion enhance the compliance. Relative changes in the effective stiffness of up to 8% imply the variation of a surface excess elastic constant of the order of 60 N m−1, much larger than the absolute value of that constant deduced from previous atomistic simulation studies of clean surfaces. Since the electrode potential affects exclusively the surface, our observations provide conclusive evidence for the impact of local stiffness variation at surfaces on the effective elastic response of nanostructures.