Upton Beall Sinclair Jr., was an American author who wrote close to 100 books in many genres. He achieved popularity in the first half of the 20th century, acquiring particular fame for his 1906 muckraking novel The Jungle.

Upton Beall Sinclair was a prolific American writer known for his affiliation with socialism and famous for his 1906 novel, The Jungle. Sinclair was born in Baltimore, but his family moved to New York when he was a child. When he was a teenager he began writing brief bits for newspapers and magazines. The Jungle was his sixth novel, and its success made him nationally famous. An exposé of the Chicago meatpacking industry, the book helped change national regulations on food preparation and earned him enough money to establish the Helicon Home Colony, a socialist community in New Jersey. Although the community only lasted about a year (it burned down in 1907), Sinclair remained committed to social causes and to exposing the dangerous effects of capitalism in realistic fiction, essays and various other writings. In 1915 he moved to California, where he ran unsuccessfully for public office several times, including running for governor as a Democrat in 1934. In the 1940s Sinclair again had popular success with the "Lanny Budd" series of novels, beginning with World's End (1940) and including the Pulitzer Prize-winning Dragon's Teeth (1942). In 1953 Sinclair moved to Arizona, where he continued to write books, including working his 1932 autobiography, American Outpost, into The Autobiography of Upton Sinclair (1962).

He is not related to Canadian broadcaster Gordon Sinclair.