This study experimentally examines how the non-dimensional thermal parameter and Reynolds number affect cavitation instability onset in a turbopump inducer using water. Based on the time-resolved static pressure measured at the inlet of the turbopump inducer, the onset cavitation number of rotating cavitation has been determined for varying Reynolds number and non-dimensional thermal parameter values. Increasing non-dimensional thermal parameter suppresses rotating cavitation and causes a monotonic decrease in the rotating cavitation onset cavitation number. At low non-dimensional thermal parameter values (e.g., 0.0125), the onset cavitation number is independent of the Reynolds number. However, at higher values of the non-dimensional thermal parameter (e.g., higher than 0.0537), the onset cavitation number increases with increasing Reynolds number. Thus, the Reynolds number promotes rotating cavitation onset. This study provides the first assessment of the independent effects of the non-dimensional thermal parameter and Reynolds number.