A 31-year-old Japanese male patient with a history of atrial fibrillation showed elevated serum levels of free thyroxine and triiodothyronine and a normal level of thyrotropin. The same abnormal hormone pattern was also found in his son. These data indicated that the index patient and the son have thyroid hormone resistance syndrome. Exon sequencing using DNA from these two patients revealed that both patients harbored a heterozygous mutation in the THRB gene: G1244C in exon 9, which results in R320P substitution. Therefore, thyroid hormone resistance syndrome caused by THRB mutation (RTHβ) was diagnosed. The mutation of the 320th arginine to proline has not been found to date. In conclusion, herein, we have described the first case of RTHβ that is associated with R320P mutation.