We report a type of quasi-phase-matched (QPM) Cerenkov third-harmonic generation (CTHG) in a periodic-poled LiTaO₃ waveguide. The CTHG results from a guided-to-guided second-harmonic generation cascaded with a guided-to-radiated sum-frequency generation (SFG) in the waveguide. In the guided-to-radiated SFG process, nonlinear interactions with participating and nonparticipating reciprocal vectors would lead to different CTHG radiations. In addition, the power and temperature detuning characters of QPM CTHG were studied. Theoretical predictions were in good agreement with experimental results.