Princess Märtha Louise of Norway (born September 22, 1971) is the only daughter of King Harald V and Queen Sonja. She is third in the line of succession to the Norwegian throne, after her brother Crown Prince Haakon and his daughter Princess Ingrid Alexandra (Märtha's niece).

In 1990 the Norwegian constitution was altered, granting cognatic primogeniture to the Norwegian throne, meaning that the eldest child, regardless of gender, takes precedence in the line of succession. This was not, however, done retroactively (as, for example, Sweden had done in 1980), meaning that the princess' younger brother still takes precedence over her in the line of succession.

Princess Märtha Louise is a certified physiotherapist, following education in Oslo and internship in Maastricht, the Netherlands. She has not practised her profession, however, choosing instead, from her fascination in traditional Norwegian folk tales as well as a love of music, to establish her own commercial entertainment business based on giving public and televised performances reciting folk tales and singing with well-known Norwegian choirs. In December 2003, she took part in Oslo Gospel Choir 's Christmas concert with a solo performance, included on the companion CD album.

On January 1, 2002, after Princess Märtha Louise started her own business, in order to work with more freedom from her constitutional role as a princess, she began paying income tax, and the King, after consulting her, issued a royal edict which removed Princess Märtha Louise's style of Royal Highness (she is entitled to the style Highness when abroad). However, she retains her place in the line of succession, and, though her activities were reduced, she still carries out some public duties on behalf of the King.

On 24 May, 2002 Princess Märtha Louise controversially married author Ari Behn (born 1972 as Ari Bjørshol; he later took his grandmother's name). On 29 April, 2003 the princess gave birth to a daughter, Maud Angelica Behn.

Unlike her aunts who do not have the style of Royal Highness, Princess Märtha Louise is not styled as Princess Märtha Louise, Mrs. Behn. She is simply Princess Märtha Louise of Norway, as the former method would imply that she lost her style because of an unequal marriage. However, it seems likely that had Princess Märtha Louise not relinquished her style of Royal Highness, she would have lost it and become Princess Märtha Louise, Mrs. Behn upon her marriage.

After several postponements due to family births and her father's illness, during which the princess took on some representation duties, Princess Märtha Louise and her husband moved to New York City in October 2004.

The princess gave birth to a girl april 8, 2005. Her second daughter has been named Leah Isadora.