Variations in the interatomic distances lead to altered d-band centre for a given metal, and provide an elegant way to control its activity towards (electro)catalytic reactions (“strain-engineering” approach). The effect of strain was rationalized within the frame of the d-band theory of Hammer and Nørskov in the late 1990s [1,2]. The authors predicted that the rate of the sluggish oxygen reduction reaction (ORR) can be enhanced on catalysts binding *OH ca. 0.10 - 0.15 eV more weakly than Pt(111), [1, 2] and this prediction was experimentally verified using a Pt3Ni(111)-skin surface.[3] Nevertheless, these predictions hardly translated to nanomaterials, because of the wide variety of catalytic sites configurations. Also, the d-band theory mostly considers catalytic surfaces in vacuum, without any effect related to the electrical double layer and adsorption/desorption processes. Hence, an in situ picture of how strain develops on Pt-based surfaces is still lacking. In this contribution, we took benefit of recent advances in Bragg Coherent Diffraction Imaging (BCDI) [4, 5] and of the fourth generation Extremely Brilliant Source of the European Synchrotron (ESRF-EBS, Grenoble, France) to map strain over Pt nanoparticles in situ. Our results show that adsorption of anions causes appearance of compressive strain at under-coordinated (edges and corners) atoms and tensile strain at highly-coordinated ({001} and {111} facets) atoms. Strain heterogeneity increases with the electrode potential and reaches as large as 0.08 %. at ORR-relevant potential. These results provide direct insights into the dynamics of Pt nanoparticles in an electrochemical environment, and have direct consequences for electrocatalysis in general, and for ORR electrocatalysis in particular.