The 20-year-old has joined on a free transfer after his deal with Arsenal expired and he will link up with Brighton's under-23 squad.
Keto joined Arsenal from Finnish side HJK Helsinki in 2014, and went into the Gunners’ under-18 squad.
Known for being an athletic shot-stopper, his finest moment with Arsenal came in the 2015/16 FA Youth Cup quarter-final against Coventry City, where he made a hat-trick of fine saves to secure a penalty shoot-out victory.
The goalkeeper has also represented Finland at youth-team level, and has three caps for the under-21s.
Albion under-23 coach Simon Rusk said: “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.”