The 23-year-old penned fresh terms at Dean Court last summer, but mounting interest in the centre-back has led the Cherries to give him an improved deal.
"I wanted something to happen because there was quite a lot of uncertainty during the summer," Cook told the club's official website.
"I had a chat with the gaffer, my parents came down and that's what made my mind up. I've been here for nearly three years now and it's my home."
Cook, who scored three goals in 40 appearances last season, joined Bournemouth from Brighton & Hove Albion in 2012.