Although lightning activity has been confirmed on Jupiter, Saturn, and Neptune through the detection of lightning-generated electromagnetic waves, its occurrence on Venus and Mars remains unclear. Here, we report observations of such a frequency-dispersed whistler detected in the ionosphere of Mars by NASA’s MAVEN spacecraft. We demonstrate the plausibility of wave propagation from the atmosphere to the spacecraft, and we show that the observed dispersion corresponds to theoretical expectations using realistic crustal magnetic field and ionospheric models. Increased attenuation at higher frequencies explains why only the low-frequency part of the whistler is observed. Our observations provide direct evidence of electromagnetic waves from an impulsive source on Mars, suggesting that electric discharges may indeed occur in the Martian atmosphere.