The Gunners have the option to make the move permanent when the temporary deal concludes.
Raya has made 161 appearances for Brentford since joining from Blackburn Rovers in the summer of 2019.
The Spain international kept 11 clean sheets in 38 Premier League games last season as Brentford finished ninth.
He is expected to provide cover and competition for Aaron Ramsdale at the Emirates Stadium.
Gunners sporting director Edu said: "We welcome David to us on a season-long loan from Brentford. David is a top-quality goalkeeper, who has consistently performed to a high level with Brentford in the Premier League.
"With David joining us we are adding quality and depth to our squad so we can perform at the highest possible level in all competitions."
Raya signed a two-year contract extension with the option for a third at Brentford before joining Mikel Arteta's side.
On the structure of the deal, Brentford director of football Phil Giles said: "I'm sure the first question that all Brentford fans will ask is why this transfer has been structured as an initial loan with Arsenal having the option to make it permanent in the future. The answer to that is simply that it enables this deal to be done more rapidly between both clubs, with all parties wanting to make this a permanent transfer as soon as practically possible, whenever that might be.
"David's contract extension is a fall-back position in the event that the transfer is not made permanent. I'm not expecting to see David back at Brentford, although if ever that happened then of course the opportunity to work with such a high-class goalkeeper for up to two more years would in many ways be an unexpected bonus for us.
"David has been a magnificent goalkeeper for us. He thoroughly deserves his move to a Champions League club and the next step in his career. In our view he is one of the best goalkeepers in the Premier League and will show himself to be one of the best in the world in the next few seasons."
