The news comes 48 hours after fellow teenager Bukayo Saka, 18, also agreed a new long-term deal with the Gunners.
The 19-year-old Martinelli - who penned his new contract exactly a year after joining Arsenal - has scored 10 goals in 26 games for the club since moving from Brazilian side Ituano last year.
He has been sidelined for the rest of the season after suffering a knee injury in training which required surgery.
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"I am very happy that Gabi has signed a new contract with us," manager Mikel Arteta told the club website https://www.arsenal.com/news/martinelli-signs-new-long-term-contract. "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."
Martinelli is the first teenager to score 10 goals in a season for Arsenal since Nicolas Anelka over 20 years ago. (Writing by Ken Ferris; Editing by Pritha Sarkar)
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