The San Diego Union-Tribune is a daily newspaper published in San Diego, California. The result of a February 2, 1992, merger of The San Diego Union, founded October 10, 1868, and the Evening Tribune, founded December 2, 1895, it refers to itself as the oldest business in San Diego County and the second-oldest newspaper in Southern California. The website Signonsandiego.com republishes much of the Union-Tribune's content as well as publishing its own material.

The newspaper is known to be "reliably conservative"; in 2008, the paper endorsed Republican party nominee Senator John McCain for president. However, during the same election cycle, the newspaper also came out against Prop 8 and thus in favor of same-sex marriage.

It was sold to the private equity group Platinum Equity on March 18, 2009.