Johan Ernst Welhaven Sars (11 October 1835 – 27 January 1917) was a Norwegian historian, publicist and editor. His main work was Udsigt over den norske Historie, four volumes issued from 1873 to 1891. He co-edited the magazines Nyt norsk Tidskrift (with Jens Lieblein) from 1877 to 1878, and Nyt Tidsskrift (with Olaf Skavlan) from 1882 to 1887. He was politically active for the Liberal Party of Norway, and among the party's most central theoreticians.
Sars was born in Kinn, Nordre Bergenhus Amt, as the son of priest and biologist Michael Sars (1805–1869) and Maren Cathrine Welhaven (1811–1898), and grew up in Manger in Søndre Bergenhus Amt. He was a brother of singer Eva Sars and zoologist Georg Ossian Sars, and a brother-in-law of explorer and scientist Fridtjof Nansen and musician Thorvald Lammers. Poet Johan Sebastian Welhaven and Elisabeth Welhaven were his uncle and aunt, and he was a cousin of architect Hjalmar Welhaven. He was not married, and lived along with his brother at their mother's place until she died in 1898. He died in Aker in 1917.
Sars attended the Bergen Cathedral School from 1849. In 1853 he moved to Christiania as a student. He initiated studies in medicine, but after having written a prize-winning treatise on the Kalmar Union, he started to study history. He spent the summers 1858 and 1859 in Copenhagen, in order to copy Norwegian documents in Danish archives. He wrote a pioneering work on Norway during the union with Denmark (Norge under Foreningen med Danmark), published in four parts between 1858 and 1865. He was appointed at the National Archival Services of Norway (Norwegian: Riksarkivet) from 1860 to 1874. After having received a scholarship he lectured at the University from 1870, where he first introduced the subject positivism, and later lectured on Norwegian history. His main work was Udsigt over den norske Historie, a continuous treatment of Norwegian history from the Viking era to contemporary times, which was published in four volumes between 1873 and 1891. The first volume of the series established Sars among the leading intellectuals in Norway, and it earned him an extraordinary professorship, after a Parliamentary decision.
Sars co-edited the magazines Nyt norsk Tidskrift (with Jens Lieblein) from 1877 to 1878, and Nyt Tidsskrift (with Olaf Skavlan) from 1882 to 1887. He took part in politics, and was active for the Liberal Party, along with Bjørnstjerne Bjørnson. After his publication of Historisk Indledning til Grundloven (Historical Introduction to the Constitution) in 1882, he was regarded among the Liberal Party's most central theoreticians. He also regarded the dissolution of the union between Sweden and Norway to be the only practical solution to the conflicts with Sweden. He wrote the work Norges politiske historie 1815–85, published between 1899 and 1904, and continued lecturing until 1911.