Abstract A thermal comfort study has been carried out in outdoor urban spaces in Singapore. The field study was carried out from August 2010 to May 2011. There were 2059 respondents from 13 different outdoor spaces participated in this study and 2036 effective questionnaire responses were collected. Thermal comfort perceptions and preferences were analyzed in this study. The neutral operative temperature occurred at 28.7 °C and preferred temperature was found to be 26.5 °C. Thermal acceptability analysis shows the acceptable operative temperature range was 26.3–31.7 °C in outdoor urban spaces in Singapore. Correlation analysis indicates that sun sensation/solar radiation has the most significant influence on human thermal sensation in outdoor spaces. This study also explores the impact of thermal adaptation on human thermal sensation in outdoor spaces, which could be useful for future researchers. Comparative analysis shows that people may expect a higher temperature in outdoor conditions than in semi-outdoor or indoor conditions in Singapore, suggesting that people in outdoor conditions could be more tolerant with the heat stress than people in indoor conditions in tropical climate.