The Scot joined the Millers in the summer of 2022 on a three-year deal following the expiry of his contract north of the border with St. Johnstone.
McCart has found opportunities hard to come by with the Millers, although injury has not helped his cause.
He has made ten appearances for the South Yorkshire club last term before being loaned out to Leyton Orient in January for the rest of the 2022-23 season.
The 26-year-old has featured three times for the Millers in 2023-24.
Millers boss Matt Taylor said: “Jamie has not had the game time he would have liked. He went on loan to Leyton Orient in January as was involved in a promotion campaign, but even then, he did not play consistently in terms of getting minutes.
"This is the opportunity to do that. I cannot speak highly enough of Jamie as a character and what he has done every single day in terms of training and his attitude. It is the best I have seen.
"But we feel he needs games to back up where his game needs to be at Championship level. It’s a local one where he does not have to worry too much about re-locating, which is always a big aspect for any footballer in trying to build that relationship with Barnsley.
"I think he will go in there and do fantastically well. He just needs to play consistent game time.
"It has been in the offing for a little while now and it’s had to get through the midweek fixtures.
"Any player who comes out on loan (from Rotherham) has a future, it’s the same with Peter Kioso. I want them all to come back in a better position or state that they went out in – to effect the level we see ourselves.
"Let’s see how they get on. I really wish Jamie all the best and wish he stays fit, first and foremost in getting the game time he warrants and deserves.”
McCart added: "I’m delighted. As soon as my agent phoned me and told me Barnsley were interested, it was kind of a no-brainer for me."