<p>Strain rate sensitivity (SRS), the strain rate dependency of material properties, is a key metric indicating a material’s suitability for use over a range of conditions. However, the effects of critical microstructural features on SRS including grain size have not been fully explored over low to high strain rate ranges. Here, physics-based molecular dynamics modeling is combined with conventional tensile testing to investigate the influence of grain size on SRS. This work explores the tensile response of nano-grained and fine-grained copper in comparison with coarse-grained copper over a range of strain rates and testing mechanisms and demonstrates the changing trend of SRS with grain size over low and high strain rate regimes. The previously established behavior of increasing strain rate sensitivity with decreasing grain size is demonstrated at strain rates below 1 × 10³&#xa0;s⁻¹, however, the reverse trend is depicted above this strain rate as seen in modeling and experimental data. In contrast, Hall–Petch strengthening, the increase in material strength with decreasing grain size until a critical grain size of ~ 10–20&#xa0;nm, is shown to occur regardless of strain rate. This work confirms the necessity of quantifying the relative strain rate range when evaluating the microstructural effects of grain size on SRS.</p> Graphical Abstract <p></p>