Various flow control techniques have been proposed to reduce skin-friction drag in turbulent flows in order to improve energy efficiency. Wall oscillation is a well-known effective method, and recent studies have demonstrated that spatially or temporally varying wall heating and cooling can mimic wall-oscillation effects through buoyancy-induced forcing. In this study, we investigate the drag reduction mechanism of turbulent channel flow subjected to periodically varying wall temperature by performing direct numerical simulations (DNS). The buoyancy force is incorporated using the Boussinesq approximation, and the amplitude is controlled by the Richardson number. Both time-periodic and streamwise traveling-wave wall temperature controls are examined. The simulations are conducted under a constant flow-rate condition at a bulk Reynolds number of Re_b = 4540. For the time-periodic control, the frequency and Richardson number are systematically varied, and a maximum drag reduction rate of 26.3% is obtained at a frequency f = 0.16 and Ri = 0.48. For the traveling-wave control, the effects of wave frequency and wavelength are explored, revealing a significantly larger drag reduction when the wave propagates in the upstream direction, which is consistent with previous observations on traveling wave controls. The maximum drag reduction rate reaches 61.4% at wavelength λ = 2π and wave speed c = -0.01 with Ri = 0.048. Flow field analyses show that the near-wall streaky structures and turbulence intensity are markedly suppressed in the drag-reduced cases. These results demonstrate that periodic wall temperature modulation is an effective strategy for turbulent drag reduction and provides further insight into buoyancy-based flow control mechanisms.