The 18-year-old was making his first competitive start for the Gunners and opened the scoring with a powerful header.
Defender Rob Holding nodded in a second before Joe Willock struck.
Reiss Nelson side-footed in a fourth before Martinelli’s superb long-range effort completed the win.
Nottingham Forest, who like Arsenal made a raft of changes for the tie, barely troubled a rampant Arsenal with their only shot on target a late Sammy Ameobi strike that was comfortably saved by Emiliano Martinez.
Martinelli makes his mark
However, back in his homeland he is viewed as having the potential to become one of Brazil’s superstars and he certainly impressed on his home debut for the Gunners.
The teenager, who made his Premier League debut with a late substitute appearance against Newcastle in Arsenal’s season opener, played futsal as a youngster and the close control and dribbling skills developed from that sport were on show against Forest.
His first goal also showed good strength and awareness as he attacked an excellent first-time ball by Calum Chambers to power home a header.
That strike made him the youngest player to score on their first start for Arsenal since Alex Oxlade-Chamberlain against Shrewsbury in September 2011.
But the best was yet to come. With the game deep into injury time, Martinelli received the ball 25 yards from goal, skipped one challenge before arrowing a wonderful strike into the top corner.
It is early days, but Martinelli showed plenty of potential to suggest he could become a hugely exciting talent for the Gunners.
More to follow.
- 3TierneySubstituted forBellerínat 77′minutes
- 32Smith RoweSubstituted forSakaat 45′minutes
- 10ÖzilSubstituted forCeballosat 71′minutes
- 24NelsonBooked at 56mins
- 18RobinsonBooked at 90mins
- 23LolleySubstituted forLawrence-Gabrielat 81′minutes
- 10CarvalhoSubstituted forAmeobiat 59′minutes
- 2RibeiroSubstituted forMightenat 78′minutes
- Darren England
- Shots on Target
Jack Robinson (Nottingham Forest) is shown the yellow card for a bad foul.
Foul by Jack Robinson (Nottingham Forest).
Attempt missed. Matthew Cash (Nottingham Forest) left footed shot from outside the box misses to the right. Assisted by Brennan Johnson.
Substitution, Nottingham Forest. Jordan Lawrence-Gabriel replaces Joe Lolley because of an injury.
Sammy Ameobi (Nottingham Forest) wins a free kick in the defensive half.
Substitution, Nottingham Forest. Alex Mighten replaces Yuri Ribeiro.
Attempt saved. Sammy Ameobi (Nottingham Forest) left footed shot from outside the box is saved in the centre of the goal.
Attempt blocked. Joe Lolley (Nottingham Forest) left footed shot from the right side of the box is blocked. Assisted by Sammy Ameobi.
Sammy Ameobi (Nottingham Forest) wins a free kick in the attacking half.
Substitution, Nottingham Forest. Sammy Ameobi replaces João Carvalho.
Tiago Silva (Nottingham Forest) wins a free kick in the defensive half.
Attempt missed. Tiago Silva (Nottingham Forest) right footed shot from outside the box misses to the right. Assisted by Jack Robinson.
Jack Robinson (Nottingham Forest) wins a free kick in the defensive half.