Like territories such as Guam or Puerto Rico, the Northern Mariana Islands are part of the US without the full status of a state.
The roughly 51,000 residents are US citizens but cannot vote in presidential elections. Crucially, some, like Saipan, also host US district courts.
Assange will appear in court at 9am local time on Wednesday.
Why is Assange heading there?
US prosecutors said Assange wanted to go to a court close to his home of Australia but not on the continental United States.
Saipan has the advantage of being relatively close to Assange’s home of Australia, roughly 3,000km (1,800 miles) south. Hawaii is more than twice as far away.
“He has to front up to charges that have been brought under US law,” said Emily Crawford, a professor at the University of Sydney’s law school.
“It had to be US territory but it had to be the US territory closest to Australia that wasn’t a US state like Hawaii.”
Saipan and the United States
After time as a colony of Spain, Germany and then Japan, the United States took control of the island in World War II.
After decades under US control, residents in 1975 voted to join the United States as a territory. The territory has a permanent delegate in the US House of Representative, although they cannot vote.
Popular tourist destination open to China
Dotted with golf courses and ringed by sandy beaches, Saipan is home to most of the Northern Mariana’s residents. Roughly 20km (12 miles) long, it only takes an hour to traverse the island.
The site of the bloody Battle of Saipan during World War 2, the island and its surroundings are dotted with memorials and wrecks popular with divers.
Tourism is the mainstay of the economy and it is popular with Korean and Chinese tourists. It is the only part of the United States that Chinese citizens can enter without a visa.
That unique status has opponents in Congress, who worry about the risk of espionage by Chinese nationals.
What happens next for Assange
US prosecutors said Assange has agreed to plead guilty to a single criminal count of conspiring to obtain and disclose classified US national defence documents in exchange for a sentence of 62 months already served.
If the judge approves his plea, Assange is expected to return to Australia after the hearing, US prosecutors said.
Julian Assange’s wife Stella Assange told Britain’s domestic Press Association news agency that her husband was paying US$500,000 (£390,000) for the flight taking him from London to Australia.
On Tuesday, the planed landed in Thailand’s capital Bangkok to refuel.
Additional reporting by Agence France-Presse