Japan's Princess Mako, the niece of the emperor, has married her college sweetheart Kei Komuro, giving up her royal title.

She said she was determined to build a happy life with her "irreplaceable" husband after a tumultuous engagement.

In an unusually frank joint news conference with her new husband, Ms Mako said "incorrect" news reports about Mr Komuro had caused her great sadness, stress and fear.

She was diagnosed with post-traumatic stress disorder (PTSD) earlier this year after a four-year engagement plagued by money scandals and intense media scrutiny.

"I'm aware that there are various views on our marriage. I feel very sorry for those (for) whom we have caused trouble," said Ms Mako, who will now be known as Mako Komuro.

She had to give up her royal title after marrying a commoner, in line with Japanese law.

"For us, marriage is a necessary choice to live while cherishing our hearts."

The two, 30, were married this morning after an official from the Imperial Household Agency (IHA), which runs the family's lives, submitted paperwork to a local office registering their marriage.

The couple broke with tradition by foregoing the rituals and ceremonies usual to royal weddings, including a reception.

Ms Mako also refused the one-off payment of about $1.3 million typically made to royal women who leave the imperial family after marriage.

Princess Mako bows as she leaves her home in Tokyo
Princess Mako (R) is hugged by her sister Princess Kako

Though Ms Mako bowed formally to her parents, her sister grabbed her shoulders and the two shared a long embrace.

Just months after the two announced their engagement at a news conference where the smiles they exchanged won the hearts of the nation, tabloids reported a financial dispute between Mr Komuro's mother and her former fiancé, with the man claiming mother and son had not repaid a debt of about $35,000.

The scandal spread to mainstream media after the IHA failed to provide a clear explanation.

In 2021, Mr Komuro issued a 24-page statement on the matter and also said he would pay a settlement.

Public opinion polls show the Japanese are divided about the marriage, and there has been at least one protest.

Analysts say the problem is that the imperial family is so idealised that not the slightest hint of trouble with things such as money or politics should touch them.

The fact that Ms Mako's father and younger brother, Hisahito, are both in the line of succession after Emperor Naruhito, whose daughter is ineligible to inherit, makes the scandal particularly damaging, said Hideya Kawanishi, an associate professor of history at Nagoya University.

"Though it's true they'll both be private citizens, Mako's younger brother will one day become emperor, so some people thought anybody with the problems he (Komuro) had shouldn't be marrying her," Kawanishi added.

The two will live in New York after Ms Mako applies for the first passport of her life.