Keith "passed peacefully ... surrounded by his family. He fought his fight with grace and courage," a post on his X account said on Tuesday.
Tributes from fellow country music stars poured in on Tuesday after news of his death.
"Today is a sad day for Country music and its fans," country music singer Jason Aldean said on Instagram.
"Toby was a huge presence in our business and someone we all looked up to and respected. You and your music will be forever remembered big man."
The popular country band Old Dominion called Keith a "true trailblazer" in an Instagram post.
"He was larger than life in so many ways and his music will live on forever!" band members wrote.
The singer, best known for his 1993 hit Should've Been a Cowboy, announced in June 2022 that six months prior he had been diagnosed with stomach cancer, saying he needed time to "breathe, recover and relax".
Keith was born in Oklahoma in 1961 and worked in the oil industry before pursuing a music career.
His career took off after his first album, which was self-titled, went platinum.
The country music star made two dozen albums and chart-topping hits like Beer for My Horses and Red Solo Cup.
His Courtesy Of The Red, White And Blue hit song was a patriotic anthem after the September 11 attacks and his father's death.
The Academy of Country Music named Keith Entertainer of the Year twice, in 2002 and 2003.
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