Nketiah, who scored a late winner against Bayern Munich last week, netted a brace at Bank of America Stadium in Charlotte, North Carolina.
The 20-year-old forward led Arsenal to another pre-season victory, scoring once in each half for the Premier League outfit before Joe Willock sealed the win.
Arsenal opened the scoring after 15 minutes thanks to a composed Nketiah.
Sead Kolasinac was released down the left before his cross was deflected to Nketiah, who kept a a cool head to finish well from inside the area to make it 1-0.
In hot conditions, Fiorentina almost drew level nine minutes before half-time, but Federico Ceccherini's header was cleared off the line by Nketiah.
The Serie A side looked threatening throughout the first 45 minutes and Arsenal goalkeeper Emiliano Martinez was called into action on the stroke of half-time, denying Riccardo Sottil after the attacker was played in behind.
A beautiful move led to another chance for Fiorentina before the break, but Dusan Vlahovic poked wide in a one-on-one with Martinez.
The encounter opened up to begin the second half, with Alexandre Lacazette curling a 20-yard free-kick over the crossbar for Arsenal in the 57th minute.
But, Arsenal doubled its lead in the 65th minute as Nketiah continued his goalscoring form.
Henrikh Mkhitaryan's cutback from the right found Lacazette, who passed to Nketiah for a simple finish.
Fiorentina was unable to find a way back into the contest amid a flurry of substitutions, Gabriel Martinelli squandering a one-on-one chance for Arsenal before the third goal arrived.
Lacazette again kept his composure before playing in Willock, who finished clinically to make it 3-0 in the 89th minute.