Whistler waves that can produce anomalous resistivity by affecting electrons' motion have been suggested as one of the mechanisms responsible for magnetic reconnection in the electron diffusion region (EDR). Such type of waves, however, has rarely been observed inside the EDR so far. In this study, we report such an observation by Magnetospheric Multiscale (MMS) mission. We find large‐amplitude whistler waves propagating away from the X line with a very small wave‐normal angle. These waves are probably generated by the perpendicular temperature anisotropy of the ~300 eV electrons inside the EDR, according to our analysis of dispersion relation and cyclotron resonance condition; they significantly affect the electron‐scale dynamics of magnetic reconnection and thus support previous simulations. Key Points Whistler waves are observed inside the EDR by MMS The whistlers are propagating away from the X line The pancake distribution of electrons in the EDR generates the whistlers