The 29-year-old defender has become a key member of the Blues' squad since arriving from Bolton in what now looks like a bargain £7million deal.
Cahill has won every major club honour since arriving at Stamford Bridge in January 2012 and is targeting more success after signing a new deal taking him to 2019.
"I'm really pleased to commit my future to Chelsea and it's great the club has put their faith in me," he told the club's official website, www.chelseafc.com.
"I have achieved a lot here and I hope I can keep on learning, helping the team and lifting trophies over the next four years."
Cahill was named in the PFA's Premier League team of the year for the 2013/14 and 2014/15 seasons, winning the league title and Capital One Cup in the latter campaign.
The Dronfield-born defender also has Champions League, Europa League and FA Cup winner's medals to his name, racking up 177 Chelsea appearances in the process.
Cahill has won 40 international caps and is England vice-captain, leading the side out for the first time in October's Euro 2016 qualifier at home to Estonia.
Source : PA
Source: PA