Abstract Chorus waves are intense whistler‐mode emissions in space that play a key role in regulating energetic electron dynamics in the hazardous radiation belt. While rising‐tone chorus has been extensively studied, the generation mechanism of falling‐tone chorus remains a long‐standing problem. Using Van Allen Probe A observations, we reveal that falling‐tone chorus in Earth’s magnetosphere originates on the downstream side of the magnetic equator along the wave propagation direction, in contrast to the upstream source of rising‐tone chorus. Guided by the typical magnetospheric plasma parameters, we perform the first simulation of falling‐tone chorus in a dipole magnetic field representative of Earth’s magnetosphere and reproduce its downstream source region. We further demonstrate that falling‐tone chorus is excited through nonlinear interactions with resonant electrons, where both wave growth and frequency chirping are triggered by resonant currents. Our study fills a critical gap in understanding the nonlinear generation of chorus waves under realistic magnetospheric conditions.