The 19-year-old striker joined Arsenal exactly one year ago today and has enjoyed an impressive debut season at the Emirates Stadium to date, scoring 10 goals and creating four more in 26 appearances across all competitions.
Martinelli has now committed his long-term future to the club, despite his previous deal still having four years left to run.
"I am very happy that Gabi has signed a new contract with us," manager Mikel Arteta told the club's official website.
"He is a very talented young player and he has impressed us with his performances, attitude and work-rate. We are looking forward to getting him back to full fitness and to watching his development at our club."