A method for measuring the zeta potential of disks is described. Combining the hydrodynamic properties of a rotating disk, the solution of Laplace's equation for the potential, and the electrokinetic boundary condition, one obtains an equation that relates the zeta potential of the disk to the streaming potential in the disk's vicinity. Theory predicts a dependence of the streaming potential on the rotation rate raised to the 3/2 power. Theory also shows that placement of one reference electrode on the axis of rotation near the disk surface and the other far from the disk is favorable. Measurement of the streaming potential of silicon oxide and indium tin oxide in contact with a solution of potassium chloride demonstrated the expected 3/2 power dependence on rotation rate. The zeta potentials calculated from the combination of the theory and experimental data agreed with published values.