Transition from laminar to turbulent flow, on a partially heated vertical wall (an adiabatic wall on top of an isothermal wall), was numerically and experimentally analyzed. The main objective was to find the location of the beginning and end of the transition based on the frequency spectra of the temperature signals obtained with a two-dimensional numerical model. The beginning of thermal transition was defined as the position where a higher frequency appeared (first harmonic) superimposed on the single laminar frequency. Further up, during transition, two or more harmonics were distinguished. The end of transition was defined as the location where no more dominant frequencies were found. A finite-volume procedure was implemented for the numerical solution. A schlieren system, thermocouples, and particle image velocimetry were used in the experiments to validate the numerical model. Four cases were considered with temperature differences of 1.1, 2.2, 4.4, and 6.6 K. The beginning and end of thermal transition in the partially heated boundary condition occurred at similar locations as in the isothermal wall, although temperatures along the adiabatic wall were considerably lower than the corresponding values in the isothermal wall.