Jonathan Greening follows Marcus Tudgay in joining the Oakwell outfit with the 33-year-old midfielder well down the Reds pecking order.
"I'm looking for Jonathan to be our handbrake on the enthusiasm we have, someone to control things, to instil an intelligent energy and bring everything together," Hill told the official BFC website.
"He has an abundance of ability, a mentality that is Premier League, it's not something that you lose."
The Tykes have also signed ex-Forest striker Emile Sinclair on loan from Peterborough.
Source: Forest MAD