Atomically thin Bi2O2Se has emerged as a new member in 2D materials with ultrahigh carrier mobility and excellent air‐stability, showing great potential for electronics and optoelectronics. In addition, its ferroelectric nature renders an ultralow thermal conductivity, making it a perfect candidate for thermoelectrics. In this work, the thermoelectric performance of 2D Bi2O2Se is investigated over a wide temperature range (20–300 K). A gate‐tunable transition from polar optical phonon (POP) scattering to piezoelectric scattering is observed, which facilitates the capacity of drastic mobility engineering in 2D Bi2O2Se. Consequently, a high power factor of more than 400 µW m−1 K−2 over an unprecedented temperature range (80–200 K) is achieved, corresponding to the persistently high mobility arising from the highly gate‐tunable scattering mechanism. This finding provides a new avenue for maximizing thermoelectric performance by changing the scattering mechanism and carrier mobility over a wide temperature range. The thermoelectric (TE) transport in few‐layer Bi2O2Se is probed, where gating can effectively modulate its scattering mechanism from polar optical phonon to piezoelectric scattering. This facilitates the capacity of drastic mobility engineering, and high TE performance over a wide range of temperature can be achieved due to the persistently high mobility arising from the highly gate‐tunable scattering mechanism.