Abstract In this paper, we provide a detailed exposition of the relativistic upper limit on wave frequency for a given wavelength in vacuum. Starting from the fundamental wave relation, where wave speed equals frequency multiplied by wavelength, we show that the maximum physically meaningful frequency occurs when a wave propagates at the invariant speed of light. This leads to the simple kinematic limit, where the maximum frequency is equal to the speed of light divided by the wavelength. This limit is exactly reached by massless waves, such as electromagnetic and gravitational radiation, and is approached asymptotically by ultrarelativistic matter waves. Mechanical waves, which depend on the properties of the medium, remain far below this limit. We illustrate these concepts with numerical examples, discuss the implications of phase and group velocities, and clarify the physical meaning of the maximum frequency in various contexts.