The muted ceremony will take place on Tuesday without any of the traditional celebrations after controversies over money in Komuro's family that have been the subject of extensive reporting in Japan.

Mako, 30, was originally due to marry her college sweetheart in 2018 but the wedding was delayed due to widespread public criticism over the relationship.

The negative media attention caused Mako to suffer post-traumatic stress disorder (PTSD), court officials announced shortly before their marriage.

After a simple civil ceremony, the couple plans to address the press once more before Mako leaves Japan to start a new life in the US, where Komuro works for a law firm.

Komuro, 30, had gone to the US to study law after the already announced wedding to Mako was cancelled in 2018. He recently finished his studies there and passed the bar exam in New York.

Mako is the niece of Japanese Emperor Naruhito.

Under law, female members of the royal family lose their royal status if they marry a commoner.

On top of the scandal surrounding Komuro's status as a regular civilian, the media have been obsessed with financial problems in Komuro's family.

There have been accusatory reports for years that Mako's future husband's mother owed money to a man to whom she had once been engaged, although there is disagreement as to whether the money was a gift or a loan.

Many Japanese were outraged by the possibility that the dispute could be settled with Mako's tax-financed royal dowry of 150 million yen ($A1.7 million).

The princess has since renounced her dowry. Komuro has said that he could use his own money to pay off his mother's debt.

Australian Associated Press