The electrical resistivity of low-carbon martensitic steels was measured to estimate the carbon concentration in the solid solution. Since electrical resistivity is influenced not only by solute carbon but also by substitutional elements, lattice defects, and second phase, the effects of these factors need to be subtracted from the total electrical resistivity to obtain an accurate solute carbon concentration via this method. Consequently, the effects of dislocations and grain boundaries were much smaller than those of solute elements, representing approximately 1–2% of the total electrical resistivity in martensitic steel. However, substitutional elements and retained austenite were found to be significantly effective. By subtracting these effects from the measured value, the change in electrical resistivity owing to solute carbon (Δρsol.C) could be formulated as a function of the carbon concentration in the solid solution of martensite (Csol) as follows:Δρsol.CmΩmm = 0.25 × Csolmass%The estimated solute carbon concentration was confirmed to correspond to the directly measured value by atom probe tomography.