0335 GMT January 19, 2019
The 26-year-old granddaughter of Emperor Akihito was scheduled to marry law firm worker Kei Komuro in November, BBC reported.
The couple announced the delay was due to a lack of time to make "sufficient preparations", Jiji Press reported.
Their decision accounted for the imperial family's busy plans next year, including the emperor's abdication.
"I am very sorry for causing a big trouble and extra burden to those who have sincerely helped our wedding," AFP quoted the princess as saying.
She will lose her title when she marries Komuro, a commoner.
An imperial household spokesperson confirmed that the couple, who met in 2012, still intend to tie the knot after important ceremonies related to the emperor's abdication and the crown prince's accession.
The household also denied the postponement was related to a recent magazine article about alleged financial problems involving Komuro's mother, according to Jiji.
In April 2019, Emperor Akihito, 84, will become the first Japanese emperor to step down from the Chrysanthemum Throne in more than 200 years.
His eldest son and uncle to Princess Mako, 57-year-old Crown Prince Naruhito, will succeed him.