The trapped electron instability, like other types of drift waves, may appear as a single mode in many linear plasma devices. Recognizing the significant variation of density gradient over the mode localization region, a nonlocal linear analysis in slab model yields eigenfunctions for Weber‐like equations. Using three‐wave, nonresonant, two‐dimensional mode coupling based on these Weber‐like eigenfunctions, two mode‐coupling equations for the fundamental and higher‐order radial harmonics are derived. The solution of these coupled equations yields the nonlinear saturation level of the unstable mode. Mode coupling to the damped higher‐order radial modes is the principal physical saturation mechanism of the unstable fundamental mode.