The 36-year-old has been with the Premier League outfit for six years, but he has slipped down the pecking order this season.
Tony Pulis has was happy to see Delap finalise a deal, telling the club's official website: "Rory has been a bedrock of the progress this club has made since he joined us.
"It's important therefore that we look after those players who have served us so well and helped us to get where we are today.
"One way or another, it's been a frustrating time for him this season, but we are delighted that he's now got the chance to go out and play regular first team football. I am sure that he will prove to be a great acquisition for Barnsley."
Delap added: "It's a big wrench to be heading out because I love the club and I have had some wonderful times here over the past six-and-a-half years.
"But I need to be playing regularly. I believe that the reason I've picked up one or two niggling injuries recently has been down to me not playing.
"I'm quite happy that a Championship side have come in for me. They're in a relegation battle, but they've put together a little run of late to raise hopes that they can climb out of trouble, so I'm looking forward to the challenge."