The 20-year-old will join the South Coast club's Under-23 squad, having been released by the Gunners at the end of the season.
Brighton U23 coach Simon Rusk told the club website: "We are delighted to welcome Hugo to the club.
"He comes here with a good grounding after his time at Arsenal, and will add some good competition for places in the goalkeeping positions.
"I look forward to working with him closely during pre-season, and to see how he develops during his time at the club."
Keto first moved to Arsenal in 2014 as a 16-year-old from HJK Helsinki, starting in the U18 squad.
Known as an athletic shot-stopper, Keto's best moment came when he saved three penalties in the quarter-final of the FA Youth Cup in 2015-16.
He also has three caps for Finland U21 and was called up to the full squad for a friendly earlier this year.
Keto played only four matches in the Premier League 2 last season.
Brighton have loaned out Robert Sanchez, who was the No 1 for the U23s last season, to League Two Forest Green Rovers next season.
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