The 22-year-old, who is expected to go straight into the squad for Saturday's Championship meeting with Sheffield Wednesday, has signed a short-term contract until June 2014 and joins for an undisclosed fee.
Frimpong endured an injury-hit career with the Gunners where, after emerging from the club's academy as a promising young midfielder, knee trouble that required surgery on three separate occasions hampered his progress.
DONE DEAL | Barnsley's new signing Emmanuel Frimpong
The Ghana international has previously been out on loan with the likes of Wolves, Charlton and Fulham although has decided to leave Arsene Wenger's side on a permanent basis having spent 13 years with the Premier League club and made 16 senior appearances.
With Aaron Ramsey ruled out for a further six weeks through injury, the Gunners boss has confirmed he is in the market for a midfielder, with Goal revealing the north Londoners are in talks with Spartak Moscow over a deal to take Kim Kallstrom on loan.