The Imperial Household Agency said the Emperor and the Empress have been mourning the demise of King Bhumibol since Thursday night, October 13.
The agency said the emperor sent Grand Chamberlain Chikao Kawai to the Royal Thai Embassy in Tokyo on Friday to express their condolences.
The imperial couple heard of the king’s passing on Thursday night after returning to the Imperial Palace from a concert they had attended with Belgian King Philippe and Queen Mathilde.
The emperor and empress had a longtime friendship with the Thai Royal Family during His Majesty’s reign, according to the agency.
King Bhumibol Adulyadej and Queen Sirikit visited Japan in 1963 and met with Emperor Hirohito, posthumously called Emperor Showa, and the following year the Japanese imperial couple visited Thailand as crown prince and crown princess on behalf of the emperor.
They travelled to a rural village by the car with His Majesty at the wheel.
After Emperor Akihito’s enthronement, he and Empress Michiko made their first overseas trip to Thailand in September 1991. They also attended the ceremony in Bangkok to celebrate the 60th anniversary of the His Majesty’s accession to the throne in 2006.
Thailand and Japan have long enjoyed stronger diplomatic relations for 129 years after the two countries signed treaty of amity and commerce during the late King Rama V.
The two countries later exchanged official and personal visits by members of the Royal families since then to strengthen their relationships.