The fee, guaranteed with no add-ons attached, comfortably surpasses the £35m Arsenal received from Liverpool for Alex Oxlade-Chamberlain in 2017 - the previous benchmark.
Al-Hilal have proposed a four-year contract for the 25-year-old, and while personal terms are still to be formally agreed, they are not expected to hold up a deal that could see Martinelli travel to Saudi Arabia as early as next week.
The move caps a gradual unravelling of Martinelli's position at Arsenal this summer. He was left out of the squad entirely for both the Community Shield win over Manchester City and the Premier League victory over Coventry, and turned down an earlier £38.4m approach from Galatasaray, whose interest he made clear he had no appetite to pursue.
Reports of interest from Serie A also came to nothing before Al-Hilal emerged as the frontrunner for his signature in recent days.
Martinelli joined Arsenal from Brazilian club Ituano in 2019 as a raw 17-year-old and went on to make 278 appearances, scoring 62 goals and playing a central role in the club's transformation from top-four hopefuls into Premier League champions under Mikel Arteta.
His departure would leave a significant gap in the club's attacking options, even with the emergence of alternatives on the left flank this season.
Should the move be completed, Martinelli would follow Ollie Watkins in joining Al-Hilal's growing collection of Premier League recruits this summer.
Martinelli's exit is expected to be one of several departures from the Emirates before Tuesday's deadline, with Gabriel Jesus and Fabio Vieira both understood to be closing in on moves of their own, to Barcelona and Hamburg respectively. For a club that has historically struggled to command significant fees for departing players, smashing its own sales record so emphatically would mark a notable shift in Arsenal's approach to squad management under sporting director Andrea Berta.