The attacking midfielder has been called up by Vicente del Bosque for Saturday's Euro 2016 qualifier with Belarus and the subsequent friendly with world champions Germany three days later.
Callejon's maiden inclusion in Spain's squad comes as reward for the superb form he has displayed this season, which has seen him surge to the top of the Serie A scoring charts with eight goals in 10 appearances.
And the 27-year-old feels he has made significant progress after joining Napoli from Real Madrid in 2013.
"Since I have been at Napoli, I've had the chance to play consistently that I wanted, gained experience and I've been able to mature in Italy," Callejon told AS.
"All of this has allowed me to score many goals and get myself noticed.
"This call-up is thanks to Rafa Benitez and Napoli. I must thank them for this honour of being able to represent my country.
"I feel a great deal of affection from the fans in Naples and that also helps me to give my best."