The £60m fee, which includes a sell-on clause on top of the guaranteed figure, becomes a club-record sale for Arsenal, surpassing the £35m they received from Liverpool for Alex Oxlade-Chamberlain back in 2017.
Given Martinelli originally cost the Gunners only around £6m when he arrived as a teenager from Brazilian side Ituano in 2019, the deal represents pure profit on the accounting books.
Martinelli, 25, has put pen to paper on a four-year contract in Riyadh, where Al-Hilal currently sit joint-top of the Saudi Pro League alongside Cristiano Ronaldo's Al-Nassr.
"Proud to be blue, proud to be Al-Hilal," the Brazil international said in an unveiling video posted on the club's social media channels.
He joins a squad managed by Simone Inzaghi that has also recently added Ollie Watkins from Aston Villa in a £51m deal and Crysencio Summerville from a relegated West Ham, with his new wages understood to dwarf the reported £180,000-a-week he was earning at the Emirates.
Martinelli had 12 months left on his Arsenal contract but had been told by manager Mikel Arteta that he would not be part of his plans for the new season, having already been left out of the squad for both of Arsenal's opening two matchday selections. That decision drew criticism from former Gunners striker Ian Wright, who said the treatment of the winger "makes me feel uncomfortable" and argued he deserved more respect regardless of the club's wish to move him on.
Despite the exit, last season underlined what Arsenal are losing: Martinelli made 53 appearances in all competitions and scored 11 goals as the club won its first Premier League title since 2004, taking his overall tally to 62 goals in 278 appearances across seven years at the club.
His departure follows that of fellow left-sided forward Leandro Trossard, who joined Besiktas in July, and takes Arsenal's total income from player sales this summer beyond £120m.
There was no indication Martinelli himself had pushed to leave, and he is understood to have turned down the option of a move to Turkey earlier in the window. Instead, discussions with Arteta over his likely role this season led him to become increasingly open to offers, culminating in his omission from the squad for both the Community Shield and the opening-day Premier League win over Coventry City.
Arsenal have since strengthened their options on the left flank with the signing of Christos Tzolis from Club Brugge, who started both of the club's opening two league victories in Martinelli's absence.