Princess Märtha of Sweden (Märtha Sofia Lovisa Dagmar Thyra; 28 March 1901 – 5 April 1954) was Crown Princess of Norway as the spouse of the future King Olav V from 1929 until her death in 1954. As Olav only became king in 1957, Märtha never became Queen of Norway. Her son, Harald V, is the current king of Norway. Princess Märtha was also an elder sister of Queen Astrid of Belgium and a maternal aunt of Grand Duchess Joséphine-Charlotte of Luxembourg and Kings Baudouin and Albert II of Belgium.

In 1940, Crown Princess Märtha and her family were immersed in World War II as Germany invaded Norway. After escaping to her home country of Sweden, and then being evacuated to America by President Franklin D. Roosevelt, she effectively advocated for Norway and did fundraising until the end of the war. In 1942, King Haakon VII of Norway, Märtha's father-in-law, invested her as a Dame Grand Cross of the Royal Norwegian Order of Saint Olav.

Märtha was born at her parents' home of Arvfurstens Palats in Stockholm on 28 March 1901, the second child of Prince Carl of Sweden, Duke of Västergötland, and his wife Princess Ingeborg of Denmark. Her father was the younger brother of King Gustav V of Sweden, making her a first cousin twice removed of the present King of Sweden, and her mother was the younger sister of King Christian X of Denmark and of King Haakon VII of Norway.

Märtha had an elder sister, Princess Margaretha of Denmark, a younger sister, Queen Astrid of the Belgians, and a younger brother, Prince Carl Bernadotte. Märtha grew up to be much more confident and outgoing and so the daughter most admired by her mother.

As a child, Märtha was taught at home by private tutors and completed in-depth courses in childcare and first aid. She and her sisters were occasionally seen shopping unaccompanied on the streets of Stockholm.

During the 1928 Summer Olympics in Amsterdam, Princess Märtha of Sweden became engaged to her first and second cousin, Prince Olav, only son and heir apparent of her uncle the King of Norway and grandson of her grandfather King Frederik VIII of Denmark's younger sister. News of the engagement was very well received: it was taken as a sign that there was no longer any tension following the dissolution of the union between Norway and Sweden. An excellent match in terms of strengthening royal ties, it was also clearly a match based on love. Initially, her younger sister, Astrid, was expected to marry Olav being younger than Olav by two years, while Märtha was two years older. Astrid was also considered more beautiful, but she instead married the future King Leopold III of Belgium.

Following a year-long engagement, she married Crown Prince Olav in Oslo Cathedral on 21 March 1929. Märtha's was the first royal wedding in Norway in 340 years. The marriage is widely believed to have been a success due in large part to their genuine mutual love. They had three children: Ragnhild (1930–2012); Astrid (b. 1932); and the much awaited heir, Harald (b. 1937).

Crown Princess Märtha soon became a popular and respected member of the royal family, later undertaking a range of official engagements and also gave many speeches, unusual for royal females in that era.