The pair met five years ago while studying at Tokyo's International Christian University.
The announcement had originally been planned for July but the couple made a decision to postpone it out of consideration for a southern region hit by heavy rains and flooding in the month.
The 25-year-old announced her engagement to former university classmate Kei Komuro, who works at a law firm, on Sunday. Of this, Princess Mako said, "From my childhood, I have spent my life, bearing in mind that when I get married, I would lose my Imperial Family member status".
Tokyo, Sept. 3 (Jiji Press)-Japanese Princess Mako and her fiancé-to-be, Komuro Kei, expressed joy at a press conference on Sunday after their informal engagement was announced by the Imperial Household Agency the same day. In return, Komuro replied that the princess has been watching over him "like the moon".
When asked about their partner's strengths, Princess Mako, 25, likened Komuro, also 25, to the sun.
In June, Japan's parliament passed a historic bill that enabled the sitting monarch, Emperor Akihito, to abdicate, passing the throne to his eldest son, Crown Prince Naruhito. "Having a family still goes beyond my imagination, but I hope to make one that is warm, comfortable and filled with smiles", she said, while everyone went "aaaaah" again. But official sources have confirmed that it delayed after a rain disaster hit western Japan.
The couple is expected to tie the knot in the same year Emperor Akihito will relinquish the throne. Mako will lose her royal status after marrying Komuro, who is a common man. If he does, he will become the first emperor to be a part of a grandchild's marriage ceremony in modern Japan.
It's a fact that concerns those who want to preserve the royal family by letting women who marry continue performing their public duties, but a government panel on the emperor's abdication avoided the divisive issue.
The couple had a long-distance relationship while studying overseas - Princess Mako in Britain and Mr Komuro in the USA - for one year.
Princess Mako is expected to Wednesday next year.