Montane reptiles are predicted to move to higher elevations in response to climate warming. However, whether upwards-shifting reptiles will be physiologically constrained by hypoxia at higher elevations remains unknown. We investigated the effects of hypoxic conditions on preferred body temperatures (T_pref) and thermal tolerance capacity of a montane lizard (Phrynocephalus vlangalii) from two populations on the Qinghai-Tibet Plateau. Lizards from 2600 m asl were exposed to O₂ levels mimicking those at 2600 m (control) and 3600 m (hypoxia treatment). Lizards from 3600 m asl were exposed to O₂ levels mimicking those at 3600 m (control) and 4600 m (hypoxia treatment). The T_pref did not differ between the control and hypoxia treatments in lizards from 2600 m. However, lizards from 3600 m selected lower body temperatures when exposed to the hypoxia treatment mimicking the O₂ level at 4600 m. Additionally, the hypoxia treatment induced lower critical thermal minimum (CT_min) in lizards from both populations, but did not affect critical thermal maximum (CT_max) in either population. Our results imply that upwards-shifting reptiles may be constrained by hypoxia if a decrease in T_pref reduces thermally-dependent fitness traits, despite no observed effect on their heat tolerance.