Objectives Historical evidence has provided information regarding disease and mortality in Napoleon Bonaparte's Grand Army, but dietary information beyond individual soldier accounts remains scarce. The purpose of this research is to reconstruct the diets of Napoleon's multiethnic army who were associated with the Russian Campaign of 1812. Materials and Methods We conducted stable carbon and nitrogen isotope ratio analysis on femoral bone collagen of 78 individuals recovered from a salvage excavation at the mass gravesite of Šiaurės miestelis in Vilnius, Lithuania. These individuals were later discovered to be Napoleonic soldiers and camp followers who participated in the 1812 Russian Campaign. Results Stable carbon isotope ratios range from −19.2‰ to −11.8‰, with a mean of −17.8‰ ± 1.5‰ (1 σ). Stable nitrogen isotope ratios range from 7.1‰ to 13.6‰, with a mean of 10.5‰ ± 1.4‰ (1 σ). Both δ13C and δ15N values show a wide range of variation. Discussion Stable isotope data indicate considerable dietary variation in this population associated with a multiethnic and socially stratified military population. Diets ranged from predominantly C3‐based to predominantly C4‐based, with varying inputs of terrestrial, freshwater, and marine animal protein. Comparison with other European populations further denotes the exceptional range of dietary variation of soldiers and camp followers in Napoleon's army.