Global gyrokinetic particle simulations and nonlinear gyrokinetic theory indicate that electron temperature gradient (ETG) instability saturates via nonlinear toroidal coupling. In such nonlinear interactions, the wave energy at the unstable high toroidal-mode number domain cascades towards the more stable lower toroidal-mode number domain via scatterings off the driven low-mode number quasi-modes. During the saturation process, there is little zonal flow generation and the radial fluctuation envelopes maintain extended structures. The nonlinear coupling process depends critically on the toroidal geometry and, as such, represents a new paradigm for the spectral cascade of drift wave turbulence in toroidal systems.