This study aimed to investigate clothing characteristics and indoor-outdoor thermal comfort of Japanese youth for an air-conditioned room in summer. A field investigation was conducted on the clothing and thermal comfort of 90 participants in Fukuoka, Japan. The main results concerning clothing were that six and five summer clothing types constituted typical summer ensembles for males and females, respectively; there was some stability in clothing characteristics over time. Concerning thermal feelings, females felt cooler or colder and felt more airflow in the room compared with males. Females also felt that conditions were significantly more comfortable and tolerable, and with less airflow, outside and were more sensitive to perceived the local differences. Males tended to adapt to the indoor environment more easily, but had difficulty in perceiving local differences in thermal feelings. Concerning the relationships between clothing and thermal feelings, the local thermal sensation was affected by the clothing conditions of different body parts and the humidity feeling was affected by clothing insulation, especially for males. Accordingly, stability, relationships, gender and local differences should be taken into account in the design solutions for summer clothing and living environments.