The highly-rated stopper was named in the party following an injury to Barcelona's Victor Valdes and is widely expected to make the final list for the tournament in Brazil.
"I'm really happy, content to be back here. I've come to enjoy doing the best I possibly can and see if I can make myself a little space in the team," Inside Spanish Football quotes De Gea as saying.
"I think the Under-21 squad has been playing really good football for a long time now and trying to do things in the same style as the main squad. There's really good young players coming through all the junior levels for Spain."
De Gea is expected to face Bolivia in Friday's friendly encounter.