Mountain Dew, stylized as Mtn Dew in some countries, is a carbonated soft drink brand produced and owned by PepsiCo. The original formula was invented in 1940 by Tennessee beverage bottlers Barney and Ally Hartman. A revised formula was created by Bill Bridgforth in 1958. The rights to this formula were obtained by the Tip Corporation of Marion, Virginia. William H. "Bill" Jones of the Tip Corporation further refined the formula, launching that version of Mountain Dew in 1961. In August 1964, the Mountain Dew brand and production rights were acquired from Tip by the Pepsi-Cola company, and the distribution expanded across the United States and Canada.

Between the 1940s and 1980s there was only one variety of Mountain Dew, which was citrus-flavored and caffeinated in most markets. Diet Mountain Dew was introduced in 1988, followed by Mountain Dew Red, which was introduced and discontinued in 1988. In 2001, a cherry-flavored variant called Code Red debuted. Expansions of the product line have continued to this day, including specialty offerings, limited time productions, region-specific and retailer-specific flavors of Mountain Dew.

Production was extended to the United Kingdom in 1996, but was phased out in 1998. A similarly named but different-tasting product, with a recipe more similar to the original American product has been sold in the U.K. under the name "Mountain Dew Energy" since 2010 and in Ireland since the spring of 2011. The product was renamed in 2014 to simply 'Mountain Dew'. As of 2017, Mountain Dew represented a 6.6% share of the carbonated soft drinks market in the U.S. Its competition includes The Coca-Cola Company's Mello Yello and Surge, and Dr Pepper Snapple Group's Sun Drop; Mountain Dew accounted for 80% of citrus soft drinks sold within the U.S. in 2010.