According to unconfirmed reports, twenty-seven-year-old Madeleine accepted Jonas Bergstrom's proposal after he got down on one knee in the royal family's summer residence, watched by both their families.
No date for the wedding has been set, but it will not happen before the nuptials of Madeleine's elder sister Crown Princess Victoria and her businessman fiance Daniel Westling, set for June 10, next year.
And neither will it precede the Swedish legislative elections, predicted for October 1, the Swedish royal household has confirmed.
Further details will be revealed on Tuesday afternoon, when Madeleine and Jonas hold a press conference at Solliden Palace, on the island of Oland.
Thirty-year-old Jonas has a master's degree in law and has worked as an associate lawyer at a firm in Stockholm since 2006. He met Princess Madeleine, the youngest of King Carl XVI Gustaf and Queen Silvia's three children, in 2002 when they were introduced by mutual friends.
Jonas invited her to dinner at an Italian restaurant in the upmarket neighbourhood of Ostermalm in Stockholm, and shortly after they began dating.