Following the 2019 coronavirus (COVID-19) outbreak, wearing masks in outdoor spaces has been strongly emphasized. However, the impacts of wearing masks on the thermal comfort of pedestrians cannot be ignored. To analyze this effect, we conducted a series of experiments with thirty subjects of the Guangzhou University. The subjects were required to walk with and without masks in different conditions and were provided with questionnaires. The subjective votes and physiological indicators of the subjects were also recorded. Physiological equivalent temperature (PET) and universal thermal climate index (UTCI) were selected for thermal comfort evaluation. The results showed that wearing masks affected the breathing and thermal sensation of the subjects. As the walking speed increased, this effect became increasingly significant. The relationships between PET/UTCI and mean thermal sensation vote with and without masks, under three walking speeds were determined. The comfort zones corresponding to PET and UTCI were narrower and the neutral values were lower for subjects with masks compared to those without masks. The walking speed of pedestrians with masks in outdoor spaces during the interim season was suggested to not exceed the upper acceptable limit of 1.63 m/s.