Ivica Olic looks to be on his way back to Hamburg, with Andre Schurrle increasingly likely to join Wolfsburg.
The former Manchester United, Middlesbrough, West Brom and Fulham man, 33, has managed only three appearances for Forest this season and joins the struggling Tykes until January 2.
Forest manager Sean O'Driscoll said: "Jonathan is a great lad, his attitude is excellent, he trains hard and Barnsley are giving him an opportunity to play on a regular basis.
"His opportunities with us have been limited since we signed Adlene Guedioura and Henri Lansbury but every time he has come into the squad he has been first class.
"I could have been selfish and kept hold of him just in case we encounter a problem but it's important to be fair to people.
"Hopefully he can go out and get some games and that will stand us in good stead if he needs to come back into our team."