The Gunners paid £3m to sign Raya on a season-long loan deal last year, with an option to make the move permanent this summer.
The Spaniard watched from the bench for his first three Arsenal matches, before he made his debut in September's 1-0 win over Everton.
From that moment on, Raya remained Mikel Arteta's first-choice goalkeeper, forcing Aaron Ramsdale to spend the majority of the campaign as his understudy.
Raya kept 16 clean sheets in 32 top-flight appearances to win the Premier League's Golden Glove award and help Arsenal mount a title challenge which ultimately ended in a second-place finish.
Arsenal trigger Raya buy option
Although his loan deal expired at the end of the campaign, there was always the expectation that Arsenal would sign Raya on a permanent transfer this summer.
According to journalist Fabrizio Romano, the Gunners have officially activated the £27m buy option in Raya's loan deal.
The Spain international has already completed the move and it is now left to Arsenal to confirm the transfer in the coming days.
Raya, who joined Brentford in 2019, will depart the West London club after recording 54 shutouts in 161 competitive appearances.
The 28-year-old is currently with the Spain squad at Euro 2024, where he has had to settle for just one appearance in four matches.
How does Raya's transfer affect Ramsdale?
The permanent signing of Raya will only reinforce Ramsdale's desire to leave the club and find regular game time elsewhere.
After being forced to settle for 11 appearances last term, Ramsdale has admitted he never wants to experience another season like the 2023-24 campaign.
While Ramsdale is likely to depart, Arsenal recently agreed a new contract to keep third-choice goalkeeper Karl Hein.
Although Hein is below Ramsdale in the pecking order, the Estonia international is expected to head out on loan for next season.
With Hein set to depart, Arsenal will have to sign a new goalkeeper before they allow Ramsdale to leave this summer.