The Spanish goalkeeper has been eager to leave Brentford since the start of the transfer window and has finally got his wish.
Tottenham Hotspur were also keen to land his signature, but Arsenal have won the race for the 27-year-old shot-stopper.
The Gunners will pay £3 million in loan fees while the deal also includes a £27m buyout clause that can be activated at the end of the season.
Mikel Arteta wanted competition for Aaron Ramsdale, and Raya is expected to provide that following a hugely impressive individual campaign with Brentford.
“I’m over the moon to be able to take this challenge in my career and make the step up,” Raya said.
“I spoke to the boss, and he sent me the values of what Arsenal are about.
“The way that the club is going, it’s growing even more, so it is a beautiful challenge to take, and I couldn’t say no.”
Raya becomes the fourth signing to link up with Arteta’s squad during the Premier League transfer window. But he may not be the last.
The Gunners have already spent £200m this summer, but they could be forced to do additional business with just over two weeks until the window closes.
Jurrien Timber, who is one of their summer acquisitions after joining in a £40m deal from Ajax, suffered a knee injury while making his Premier League debut against Nottingham Forest on Saturday.
The injury could sideline the Netherlands international for months, which could force Arsenal to splash the cash again.
The Gunners have already been linked with a move for Manchester City centre-back Aymeric Laporte, who could exit the Etihad Stadium this summer.
Josko Gvardiol’s arrival has pushed the Spaniard further down in the pecking order, although the Cityzens are unlikely to sanction another deal with their title rivals.
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