Abstract The Z‐mode, comprising fast and slow branches, is a ubiquitous electromagnetic wave in planetary magnetospheres. While fast Z‐mode generation is attributed to electron cyclotron maser instability, the slow Z‐mode mechanism remains unresolved. Van Allen Probes observations discover that slow Z‐mode waves can emerge as second harmonics of coexisting chorus emissions. We derive a concise and general formulation that explicitly links fundamental waves to their second harmonics. Combining with the correlated chorus waves data, our formulation successfully explains three defining characteristics of slow Z‐mode: spectral range, wave amplitude, and rising‐tone patterns. This demonstrates that the second harmonic serves as a previously unrecognized generation pathway for slow Z‐mode, which has broad implications for understanding wave dynamics across diverse magnetospheric environments.