The 52-year-old wife of Crown Prince Haakon, the heir to the Norwegian throne, was diagnosed with pulmonary fibrosis in 2018, a chronic disease that causes scarring in the lungs and leads to a reduced oxygen uptake.
"Like all newly transplanted patients, the crown princess will remain at the hospital for several weeks to come," Oslo University Hospital Professor Are Holm said in a statement provided by the palace on Wednesday.
The surgery comes at a strained time for the royal family: Mette-Marit's 29-year-old son from a previous relationship, Marius Borg Hoiby, on Monday was convicted of rape and domestic violence and sentenced to four years in prison.
Oslo University Hospital on June 5 said Mette-Marit had been placed on a waiting list for a lung transplant after a significant deterioration in her health that likely gave her only a year to live without the surgery.
The crown prince and crown princess thanked the public for the warm and kind greetings they had received recently, the palace said.
The next public update on her health is only expected when she is discharged from hospital, it added.
Crown Prince Haakon in December said the family had noticed a change in Mette-Marit's condition and that she was struggling more to breathe.
Prime Minister Jonas Gahr Stoere had praised the crown princess for being open about her illness, saying this could help others suffering from similar conditions.
Mette-Marit was 25, an unmarried single mother and a commoner when she met Haakon at a music festival in 1999 - the beginning of an unlikely royal romance that started with a media furore and ended up winning over much of the nation.
But support for Norway's monarchy has taken a hit in 2026, opinion polls have shown, as several scandals hit at once.
Hoiby's conviction on Monday followed a highly publicised trial that dominated headlines.
Meanwhile, the crown princess has apologised to the king and queen for her contact with late US sex offender Jeffrey Epstein, whom she had described as a friend, though she cut ties several years before his death in 2019.
A Norstat survey in February, conducted during Hoiby's trial, showed support for retaining the monarchy falling to a record low of 60 per cent before recovering slightly to 64 per cent in May.
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