Summary: We derive an uncertainty relation for two unitary operators which obey a commutation relation of the form \(UV=e^{i\phi}VU\). Its most important application is to constrain how much a quantum state can be localized simultaneously in two mutually unbiased bases related by a discrete Fourier transform. It provides an uncertainty relation which smoothly interpolates between the well-known cases of the Pauli operators in two dimensions and the continuous position and momentum variables. This work also provides an uncertainty relation for modular variables, and could find applications in signal processing. In the finite-dimensional case the minimum uncertainty states, discrete analogues of coherent and squeezed states, are minimum energy solutions of Harper’s equation, a discrete version of the harmonic oscillator equation.