In 2016, Bournemouth paid £7m to sign Cook from Leeds United and the 21-year-old has since gone on to make 35 appearances in the Premier League.
Cook's form this season has been rewarded with an England debut in March, and the Cherries have moved to tie down the player until 2022.
"This new contract is a really proud moment for me and the icing on the cake for the year we've had and all the hard work that's gone on behind the scenes at the club," Cook told the club's official website.
"It came as a nice surprise. I was just concentrating on my football and playing games, but when it was spoken about I couldn't wait to get it signed. I'm really enjoying my football here and I'm looking forward to the future with AFC Bournemouth."
Cook is currently on standby for England ahead of the World Cup, which begins next month.