The Take That songwriter will replace Lionel Richie, after the 'Hello' singer was forced to cancel his planned appearance on September 11.
Barlow told BBC Radio 2: "It's a pleasure to step in last minute and cover for Mr Richie. Chris [Evans] called me and said Lionel had had to pull out because he was ill and they had a hole in the show, so I was more than happy to help out."
Evans added: "Everyone loves a hero… and ours is Gary Barlow. Gary has stepped into a void that was opened up when Lionel Richie had to cancel his appearance at this Sunday's Live in Hyde Park.
"He had a real problem with his throat, under doctors' orders, couldn't do anything about it. He was still in America, he couldn't get on a plane, he had to stop recording his new album which he was gonna do. It just wasn't gonna happen."
Nile Rodgers's Chic were also added to the bill yesterday and join a lineup which includes Lenny Kravitz, Will Young, James Blunt, Beverley Knight, Imelda May, Jools Holland with Sandie Shaw and others.
Tickets are still available at £32.50. Doors open at 2.30pm and the acts will play from 3.30pm-10pm.
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Watch Take That's video for 'The Flood' below: