Fans at the event described seeing the singer drop to the ground and begin shaking mid-way through a song.
A spokeswoman for South Central Ambulance Service said: “An ambulance was called at 9.30pm to an incident in George St in Oxford.
“We have a crew on scene and one patient has been taken to hospital.”
A paramedic said the star’s condition was stable.
It comes after the Driving Home For Christmas hit-maker, 66, suffered a stroke last year sparking fears for his health.
Darren Fewins, who had been watching the show with his wife, told the Press Association the musician had been on stage for 40 to 45 minutes before he fell backwards.
“He was standing by the microphone and he had the guitar in his hand and he kept shaking his left hand as if there was something not quite right with it,” the 46-year-old said.
“He walked forward to the microphone and he was trying to grab the microphone with his left hand, and it was swinging from side-to-side.
“I thought at first there was something wrong with it and he was trying to keep it steady. He then took three or four steps backwards.
“Then he just collapsed on the stage backwards. There was no one there to catch him, he just fell into a clump.
“He was on his back for about two minutes before they brought the curtain down and then that was it.”
The evening had been brilliant up till now with Chris on top form but Chris Rea has just collapsed on stage at the New Theatre Oxford where I'm watching him.— CörnishDarrenFewins (@Darren_Fewins) December 9, 2017
Fell backwards in a clump on the stage.
Really doesn't look good.
I'm quite upset. pic.twitter.com/niA4O7aUAz
One fan posted on Twitter: “The evening had been brilliant up till now with Chris on top form but Chris Rea has just collapsed on stage at the New Theatre Oxford where I’m watching him.
“Fell backwards in a clump on the stage. Really doesn’t look good. I’m quite upset.”
The New Theatre in Oxford refused to comment.
Chris was in the middle of his The Road Songs for Lovers European Tour.
Born in March 1951, Rea is 66 and is famed for his husky voice and was once described as a “gravel-voiced guitar stalwart”.
In 2000 Rea underwent surgery after being diagnosed with pancreatic cancer and has suffered from diabetes and other health issues ever since.
Rea has continued to tour and in 2016 he suffered a stroke which left him with reduced movement in his arms and fingers as well as slightly slurred speech.