The influential sister of North Korean leader Kim Jong-un is to attend Friday’s opening ceremony of the Winter Olympic Games, which are being held in the South, ministers in Seoul say.
Kim Yo-jong is the youngest daughter of late leader Kim Jong-il and her role was strengthened last year when she was promoted to the politburo.
Both Koreas will march under one flag at the opening ceremony.
The North’s participation has been seen as a thawing of bilateral ties.
However, experts say it is unlikely to have any impact on the North’s nuclear ambitions.
The US believes the North is using the Olympics for propaganda purposes and is sending Vice-President Mike Pence to the opening ceremony in Pyeongchang to counter it.
“We’re travelling to the Olympics to make sure that North Korea doesn’t use the powerful symbolism in the backdrop of the Winter Olympics to paper over the truth about their regime,” he said.
Kim Yo-jong, who shares the same mother as Kim Jong-un, will accompany the North’s ceremonial head of state, Kim Yong-nam, whose attendance was announced at the weekend.
A message from her brother?
Analysis: BBC’s Laura Bicker in Seoul
This is a huge surprise. There had been speculation Kim Yo-jong might be part of the delegation but few thought it would actually happen. She will be the first immediate Kim family member to cross the border. It is being seen here as a sign that Kim Jong-un is serious about improving ties with South Korea.
Kim Yo-jong is one of Kim Jong-un’s closest aides and some are speculating that she might be bringing a message from her brother.
But there could be a number of obstacles to overcome if she is to make it to Pyeongchang. She is targeted by US sanctions for alleged human rights abuses, although she is not on the UN Security Council travel blacklist.
There is also the question of how she will get to the Games. Seoul has had to request special permission from the US and others in the international community to allow North Korean athletes and performers to travel south by bus and ferry. They may have to do so again and this time it will be for a member of Kim Jong-un’s own family.
Most of the 280-member North Korean delegation, including a team of cheerleaders, arrived in the South on Wednesday.
The delegation, led by North Korean Sports Minister Kim Il-guk, includes 229 cheerleaders, four officials from the National Olympic Committee, 26 taekwondo demonstrators and 21 journalists.
The team arrived via a western border at 09:28 local time (00:26 GMT), the Yonhap news agency reported.
Only 10 athletes will compete for the North at the Games, along with another 12 as part of a unified Korean women’s ice hockey team.
Believed born in 1987, Kim Yo-jong is said to be very close to Kim Jong-un, who is about four years older than her. The two of them lived and studied in Berne, Switzerland, at the same time.
She is reportedly married to the son of Choe Ryong-hae, the powerful party secretary, and is a senior official in the ruling Workers’ Party.
She has been in the spotlight sporadically in recent years, with her main job being to protect her brother’s image via her role in the party’s propaganda department.
She remains blacklisted by the US over alleged links to human rights abuses in North Korea.