The 24-year-old came one for the final ten minutes of the game at the Emirates on Sunday (September 21), replacing Jurrien Timber.
Martinelli latched on to a ball over the top from Eberechi Eze, lobbing a helpless Gianluigi Donnarumma who was well off his line.
It’s not the first time the Brazilian has come off the bench to make an almost immediate impact, he also scored and provided an assist in the 2-0 Champions League win over Athletic Club in midweek.
Speaking after the games, Martinelli admitted he’s out to reclaim his place in Mikel Arteta’s starting eleven.
"We wanted to win the game," Martinelli told BBC Sport. "I think we created a lot of chances during the game and were just a bit unlucky in certain moments.
"(I'm) happy with the goal, but we didn't want to draw the game, we wanted to win it, so let's keep working to win these (types of games).
"I work for those moments. I leave (selection) for the gaffer; he knows what he does. I just do my best to help this football club.
"Of course, I don't want to be on the bench. I want to play 90 minutes every single game, but (Arteta) knows what he does, he's the gaffer and everyone trusts him. We're really happy with him."