AbstractHard carbon (HC) is the negative electrode (anode) material of choice for sodium‐ion batteries (SIBs). Despite its advantages in terms of cost and sustainability, a comprehensive understanding of its microstructure is not complete yet, thus hindering a rational design of high‐performance HC electrodes. In this study, rather than investigating how the precursor and synthesis method influence on the electrochemical properties of HC anodes, we examine the microstructure and surface chemistry of three optimized HC anodes obtained from different precursors by using different synthesis routes. The main goal is to evaluate the influence of the final materials properties (in their optimized state) on the electrochemical reactivity in lithium and sodium cells after a comprehensive structural characterization performed by means of X‐ray photoelectron spectroscopy (XPS), wide‐angle X‐ray scattering (WAXS), Raman spectroscopy, scanning electron microscopy (SEM), and gas sorption measurements. The different electrochemical performance observed in terms of cycling stability and rate capability, and the stability of the solid electrolyte interphase (SEI) formed on the various HCs have been comprehensively investigated. A correlation of the material properties with their electrochemical response upon sodium and lithium uptake and release is clarified. By comparing the Na‐ and Li‐ion storage behavior, a structure‐function relation is identified.