Ostrom was the first woman to win the prize since it was founded in 1968, and the fifth woman to win a Nobel award this year - a Nobel record.
The Royal Swedish Academy of Sciences cited Ostrom "for her analysis of economic governance," saying her work had demonstrated how common property can be successfully managed by groups using it.
Williamson, the academy said, developed a theory where business firms serve as structures for conflict resolution.
"Over the last three decades, these seminal contributions have advanced economic governance research from the fringe to the forefront of scientific attention," the academy said.
The economics prize was the last Nobel award to be announced this year. It's not one of the original Nobel Prizes, but was created by the Swedish central bank in Alfred Nobel's memory.
Nobel Prize winners receive 10 million Swedish kronor ($1.4 million), a gold medal and diploma from the Swedish king on Dec. 10, the anniversary of Nobel's death in 1896.
Last week, American scientists Elizabeth H. Blackburn, Carol W. Greider and Jack W. Szostak shared the Nobel Prize in medicine for discovering a key mechanism in the genetic operations of cells, an insight that has inspired new lines of research into cancer.
The physics prize was split between Charles K. Kao, who helped develop fiberoptic cable, and Americans Willard S. Boyle and George E. Smith who invented the "eye" in digital cameras.
Americans Venkatraman Ramakrishnan and Thomas Steitz and Ada Yonath of Israel shared the chemistry prize for their atom-by-atom description of ribosomes.
Romanian-born German writer Herta Mueller won the literature prize and on Friday, President Obama was named this year's winner of the peace prize.