A new frequency-to-phase mapping technique for measuring a radio-frequency (RF) signal frequency is presented. The concept is based on generating two low-frequency signals where their phase difference is dependent on the input RF signal frequency. Hence, the input RF signal frequency can be determined by using a low-cost low-frequency electronic phase detector to measure the phase difference between the two low-frequency signals. The technique can measure the frequency of an RF signal instantaneously and has a wide frequency measurement range. The proposed frequency-to-phase-mapping-based instantaneous frequency measurement system is experimentally verified over the 5 to 20 GHz frequency measurement range with errors of less than ±0.2 GHz.