Abstract The measurement of cell membrane potential (V _m) is important for understanding ion channel function. V _m plays a role in several routine cellular functions and diseases, particularly in excitable cells such as muscle and nerve. However, measuring V _m is difficult, relying either on labour-intensive direct measurement of single cells (intracellular electrodes, patch clamp) or indirect measurement of fluorescence intensity, using V _m-sensitive labels. Here we demonstrate a direct measurement technique based on determination of the cell’s ζ-potential, the electrical potential at the hydrodynamic shear plane, approximately 1 nm beyond the cell surface. We demonstrate this principle using excitable H9c2 cardiomyoblasts, measured in both polarised and depolarised states, before and after extracellular intervention to alter cell ion concentration. Given widespread availability of ζ-potential measurement apparatus (most typically in chemistry and materials science settings), this offers a new method of measuring V _m without the need for fluorescence measurements or calibration curves.