Rice is sensitive to high temperature, especially at the reproductive stage, which causes spikelet sterility and yield losses. To identify heat-tolerant genetic resources for future genetic studies and breeding, we screened 455 IRRI Genebank accessions from hot rice growing regions in the field in the hot season, and 200 accessions with high spikelet fertility were selected for phenotyping in temperature-controlled outdoor growth chambers with 38/21 C day/night temperature and 70/75% day/night relative humidity (RH). From the 200 accessions, 28 were selected and reconfirmed along with 12 potential heat-tolerant accessions from IRRI breeding programs. Finally, 23 accessions were selected as potential donors for heat tolerance. The heat tolerance of the 23 accessions was also evaluated by exposing them to controlled high temperature during booting and flowering stages. High-temperature treatment of 39 ºC is suitable for screening heat tolerance at booting stage, while 38 ºC is suitable for flowering-stage evaluation. Dular and Todorokiwase are tolerant at the booting stage, while Milyang23 and IR2006-P12-12-2-2 are tolerant at the flowering stage. Giza178 is tolerant at both booting and flowering stages. The heat-tolerant varieties identified and the phenotyping protocols developed in this study could be used in future genetic studies and breeding programs focusing on heat tolerance.