The Brazilian forward has been secured on a long-term contract, with the deal subject to the completion of regulatory processes.
Martinelli, 18, will shortly join up with his team-mates in north London for pre-season training.
And it begins Arsenal’s summer spending as he arrives for a reported £5m.
Unai Emery has a limited budget to play with ahead of the 2019/20 campaign.
But Martinelli will be joined by further additions prior to the start if the new season.
The likes of Crystal Palace’s Wilfried Zaha, Saint-Etienne’s William Saliba and Celtic’s Kieran Tierney are all being pursued.
Bids have been lodged for all three players, and it is the latter who Arsenal fans appear to want most.
Supporters reacted to the Martinelli announcement on Twitter by bombarding the Tweet with replies about Tierney.
“Finally, NOW SIGN TIERNEY”
The Scotland international left-back is available for £25m and is a top target for Emery.
However, Napoli are also very interested in signing the Celtic Park favourite.
While there is competition from the Serie A giants, Arsenal are still in the running to complete a deal, having had £15m and £18m offered rejected so far.
Gunners fans remain hopeful, and they made their desire clear, largely insisting Tierney is of greater priority to the club than Zaha or Saliba.
In reply to the Martinelli Tweet, one fan posted: “Finally, NOW SIGN TIERNEY”.
Another said: “NOW ANNOUNCE TIERNEY.”
A third added: “NOW TIERNEY”.
Fans appeared to agree with the Tierney claims as those Tweets had the most likes from fellow supporters.
Reports have suggested Arsenal will step up their pursuit of Tierney this week as Napoli are hovering, with a £20m move being lined up.