The "Okie From Muskogee" singer had been suffering from pneumonia and spent 11 days in the hospital earlier this year, according to TMZ. Last week he canceled concert dates with Willie Nelson, with whom he collaborated on an album last year, after his health worsened.
Haggard was born April 6, 1937, in Oildale, Calif. He was the son of Depression-era migrants from Oklahoma, which partially inspired his most famous song. His other hits included "Mama Tried," "If We Make It Through December" and "The Fightin' Side of Me." Along with Buck Owens, he was considered the creator of the "Bakersfield sound" and was one of the central figures of "outlaw country," a rough-edged and gritty style of country music.
As a young man, he served time for attempted robbery in California's notorious San Quentin Prison, and began his musical career after his release in 1960. In 1972, then-governor Ronald Reagan expunged his record.
He is survived by his fifth wife, Theresa, and six children.
Watch Merle Haggard perform "Okie From Muskogee" during a 1969 TV appearance: