Upton Beall Sinclair Jr. (September 20, 1878 – November 25, 1968) was an American writer, muckraker, political activist, and the 1934 Democratic Party nominee for governor of California. He authored nearly 100 books and other works across several genres. Sinclair's writings garnered widespread popularity and recognition in the first half of the 20th century, and he was awarded the Pulitzer Prize for Fiction in 1943.

In 1906, Sinclair rose to particular fame with his landmark muckraking novel, The Jungle, which exposed labor and sanitary conditions in the U.S. meatpacking industry, triggering a public uproar. This uproar contributed to the enactment of the 1906 Pure Food and Drug Act and the Meat Inspection Act. His 1919 publication, The Brass Check, was a groundbreaking exposé on American journalism that highlighted the issues of yellow journalism and the constraints on the 'free press' in the United States. Four years after its publication, the first code of ethics for journalists was established.

Time magazine once described him as "a man with every gift except humor and silence." He is also famous for the quote: "It is difficult to get a man to understand something, when his salary depends upon his not understanding it." This quote was frequently used by Sinclair in speeches and in his book about his gubernatorial campaign to explain the indifference of newspaper editors and publishers to his proposals for old age pensions and other progressive reforms.

Many of Sinclair's novels serve as historical narratives, offering perspectives from both the working man and the industrialist. His works such as King Coal (1917), The Coal War (published posthumously), Oil! (1927), and The Flivver King (1937) vividly depict the working conditions in the coal, oil, and auto industries of his time. The Flivver King narrates the rise and fall of Henry Ford, touching upon his "wage reform" and eventual descent into antisemitism.

As an outspoken socialist, Sinclair unsuccessfully ran for Congress as a nominee from the Socialist Party and for governor of California under the Democratic Party during the Great Depression, championing the End Poverty in California campaign.