The captain has two years to run on his contract. However, the decision will not have an impact on a deal to bring De Gea to the club. De Gea is out of contract in 2016 and Madrid want to sign him this summer.
Despite the fact that Iker will start in the last match of the season at the Santiago Bernabéu it was seen as a possible farewell to the supporters.
Casillas held a meeting on Tuesday with president Florentino Perez and received confirmation that he will continue for next season at least.