The 25-year-old eldest granddaughter of Emperor Akihito will become engaged to law firm worker Kei Komuro, also 25, whom she met while studying together.
Japan's imperial law requires a princess to leave the royal family after marrying a commoner.
The move is expected to reignite debate on royal succession, with the emperor also possibly abdicating soon.
Princess Mako and Kei Komuro met in 2012 at a restaurant, when they were both studying at the International Christian University in Tokyo.
Mr Komuro once worked as "Prince of the Sea", promoting tourism in Japan
What happens now?
The Imperial Household told local media that plans were under way for the princess's engagement.
According to the Associated Press news agency a public announcement will be made, and then a wedding date will be set. AP said the couple would also make a formal report to the emperor and empress.
The engagement will only be official after a ceremonial exchange of gifts, local media said.
Asked about their engagement plans, Mr Komuro on Wednesday was quoted as saying: "Now is not the time for me to comment, but I want to speak at the right time."
Has this ever happened before?
Yes, Princess Mako's aunt Princess Sayako married a commoner in 2005 - the first time a Japanese royal became a commoner.
Former princess Sayako had to learn how to drive and buy furniture
Her wedding to an urban planner for the Tokyo city government, was described as a low key event. And Sayako was left to adjust to her more humble surrounding.
She moved into a one-bedroom apartment, had to learn how to drive, shop in a supermarket and buy furniture.
Princess Sayako is the only daughter of Emperor Akihito.
How does this affect the royal family?
Emperor Akihito, 83, hinted last August that he wanted to stand down, saying his age could interfere with his duties.
No Japanese emperor has abdicated for two centuries and the law currently does not allow it, but Japan is currently considering legal changes to allow the emperor to abdicate.
However, the new legislation is expected to leave unchanged a males-only succession law - which has been at the centre of debate for many years.
Because of that law there are only four heirs to the Chrysanthemum Throne: Akihito's sons Crown Prince Naruhito and Prince Fumihito, Prince Hisahito (Fumihito's son) and the emperor's younger brother, Prince Masahito.
After news of Princess Mako's upcoming engagement broke, Cabinet Secretary Yoshihide Suga was quoted by Reuters news agency as saying: "There is no change in our view to proceed with consideration of steps to ensure stable imperial succession."