The 18-year-old, who can play as a second striker or a winger, was signed for a fee in the region of six million pounds according to British media reports.
"Brazilian forward Gabriel Martinelli is joining us from Ituano Futebol Clube on a long-term contract," Arsenal said in a statement on their website https://www.arsenal.com/news/gabriel-martinelli-join-club.
"The deal is subject to the completion of regulatory processes."
The teenager was called up by Brazil manager Tite in May for a training camp ahead of the Copa America but did not make the cut for the final 23-man squad.
(Reporting by Rohith Nair in Bengaluru; Editing by Ken Ferris)