Japanese Emperor Akihito and Empress Michiko arrive in Thailand on Sunday to bid an official farewell to the late King Bhumibol Adulyadej. The king's friendship with the imperial couple has contributed to bilateral relations between the palaces for over 50 years.
The couple are paying a private visit to the country following their state visit to Vietnam.
"We are treating this as a significant event," said Don Pramudwinai, minister of foreign affairs.
"The imperial visit doesn't only reflect the special connection between the two royal families. Emperor Akihito will also be the first head of state to be received by His Majesty the King Rama X."
The most recent monarchs' ties date back to Emperor Hirohito's era when he met King Bhumibol and Her Majesty Queen Sirikit on their 1963 visit to Japan.
On the emperor's behalf, the then-crown prince Akihito and princess Michiko went to Thailand the following year.
When Emperor Akihito acceded to the throne in 1989, he chose Thailand as the first destination for his 1991 Southeast Asia tour.
The emperor's most recent trip to Thailand took place in 2006, marking the 60th anniversary of King Bhumibol's accession to the throne. Several royal families from around the world also attended the celebrations.
"I remember the Japanese emperor and empress in a motorcade driving by Thai well-wishers lining the side of the road to get a glimpse," said Mr Don, then working as a Foreign Ministry liaison officer. "The emperor and empress lowered their car window and waved to the people.
"They waved their flags and cheered loudly with delight. As Thais, we felt appreciated with the royal couple's gesture.
"When the Thai prince and princess visited Japan, the emperor and empress often granted them an audience as if they were welcoming home the children of their friends, and vice versa."
After King Bhumibol's passing last year, the imperial couple declared a private mourning period of three days.
An important symbol of the royal ties is tilapia fish or pla nil. In 1965, Emperor Akihito, then crown prince, sent several dozen tilapia fish as a gift to the Thai king for breeding.
"The emperor wanted to help improve the malnutrition of Thais in rural areas by giving out pla nil with high protein," said Kitti Prasirtsuk, director of the Institute of East Asian Studies at Thammasat University.
The first attempts to breed pla nil in Thailand ended unsuccessfully with 40 fish dying. However, King Bhumibol proceeded to breed the remaining fish in his palace pond.
One year later, he gave 10,000 young tilapia fish to the Department of Fisheries, making the fish a lasting local staple.
According to Mr Don, the Thai government has since shared its pla nil with other countries.
"Pla nil is a symbol of the humanity that the two royal families have contributed," said Mr Kitti. "We will continue passing on the compassion of the royal family by sharing our pla nil."
Krairoek Nana, a historian, said the Thai-Japanese royal relationship could be traced back as far as King Rama V's era when they would exchange gifts such as silk.
"King Rama V was interested in Japanese technology during the Meiji era when Thailand was modernising as a country," he said.