The Belgium international has been installed as the London club's first-choice goalkeeper this season at the expense of Petr Cech.
Courtois spent the last three seasons on loan at Atletico Madrid, but sees his future at Chelsea after breaking into Jose Mourinho's first team.
He told the club's official website: "I'm very happy with this new deal, it's really nice to have signed this new contract for five years.
"My future was already sorted but it's nice to know I'm going to be here for another five years and now I can just concentrate on playing games."
The 22-year-old joined Chelsea from Genk for an undisclosed fee in 2011 on a five-year contract and was viewed as Cech's long-term successor.
Courtois was soon loaned out to Atletico and became a firm favourite over three successful campaigns in Madrid; during which he won La Liga, the UEFA Europa League, the Copa del Rey and the UEFA Super Cup.
He started all five games for Belgium at the World Cup in Brazil and was given the nod by Mourinho to take over from the long-serving Cech for each of Chelsea's first three Premier League games so far this season.
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