The conversion of natural habitats to human land uses often increases local temperatures, creating novel thermal environments for species. The variable responses of ectotherms to habitat conversion, where some species decline while others persist, can partly be explained by variation among species in their thermal niches. However, few studies have examined thermal niche variation within species and across forest-land use ecotones, information that could provide clues about the capacity of species to adapt to changing temperatures. Here, we quantify individual-level variation in thermal traits of the tropical poison frog, Oophaga pumilio, in thermally contrasting habitats. Specifically, we examined local environmental temperatures, field body temperatures (T b ), preferred body temperatures (T pref ), critical thermal maxima (CT max ), and thermal safety margins (TSM) of individuals from warm, converted habitats and cool forests. We found that frogs from converted habitats exhibited greater mean T b and T pref than those from forests. In contrast, CT max and TSM did not differ significantly between habitats. However, CT max did increase moderately with increasing T b , suggesting that changes in CT max may be driven by microscale temperature exposure within habitats rather than by mean habitat conditions. Although O. pumilio exhibited moderate divergence in T pref , CT max appears to be less labile between habitats, possibly due to the ability of frogs in converted habitats to maintain their T b below air temperatures that reach or exceed CT max . Selective pressures on thermal tolerances may increase, however, with the loss of buffering microhabitats and increased frequency of extreme temperatures expected under future habitat degradation and climate warming.Abstract in Spanish is available with online material. K E Y W O R D Samphibians, body temperature, Costa Rica, CT max , ectotherm, habitat loss, land use, tropical wet forest S U PP O RTI N G I N FO R M ATI O NAdditional supporting information may be found online in the Supporting Information section at the end of the article.