Thermal preference is a laboratory-based measurement that is assumed to represent the body temperature an ectotherm would select in the wild. A proliferation of data on thermal preference has occurred recently because many studies use the metric for predicting the potential for species to cope with rapid changes in climate. However, the ability of an individual to attain physiological optimal body temperatures may be constrained by intra-specific competition for basking sites. To introduce a realistic constraint in thermal preference measurements, we measured preferred body temperature (T_pref) in tree lizards (Urosaurus ornatus) in (i) solo thermal gradient trials and in paired trials with (ii) individuals of the opposite sex, (iii) individuals of the same sex and same throat color morph, and (iv) individuals of the same sex and different throat color morph. We found clear evidence that conspecifics influenced T_pref in U. ornatus. When two males shared a gradient, T_pref values decreased significantly compared to solo values, regardless of if the competing males were of the same or different color morph. Males with partially blue throats were displaced the least, indicating dominance in thermoregulatory interactions, and males with non-blue throats (yellow or orange morphs) were displaced the most. Females were significantly displaced from their solo T_pref values when sharing a gradient with females of the same color morph and with males, while males were not displaced by females. Including realistic constraints in T_pref measurements likely provides a more accurate depiction of thermoregulation in tree lizards, and similar measurements with realistic constraints introduced could be used to inform more accurate models predicting species responses to climate change.