48-year-old Westerwelle and businessman Michael Mronz entered the wedlock on Friday when Bonn's Mayor Juergen Nymptsch personally performed the ceremony as the registrar, local media reports said.
Neither Chancellor Angela Merkel nor any of Westerwelle's Cabinet colleagues were present when the Foreign Minister gave his 'ja' or yes to Mronz, 43, and the ceremony was attended only by a handful of closest family members and friends of the couple, the reports said.
Westerwelle and Mronz, a promoter of sporting events, later celebrated the formalisation of their partnership in a restaurant in Cologne.
They met for the first time at a horse racing championship in the city of Aacnen in 2003 and since then they have been living as a couple.
In Berlin, a government spokesman said last night that Chancellor Merkel telephoned Westerwelle shortly after his wedding to wish the two men good luck.
Westerwelle's liberal Free Democratic Party (FDP) is a junior partner in Merkel's centre-right coalition, which came to power after the general elections in September last year.
Germany eased restrictions on same sex marriage in 2001 and allowed gay and lesbian couples to form "registered partnerships," which give them the same rights in many areas as heterosexual couples.
However, homosexual marriages are still not treated as equal to a marriage between a man and a woman.
Westerwelle has set new standards in German politics by becoming the first openly gay politician to lead a major political party and later assuming one of the highest offices in this country.
After nearly a year at the helm of Germany's diplomacy, he earned respect from his critics, who had expressed doubts about the suitability of a gay politician as the nation's chief diplomat.