The 40-year-old Take That star will be performing at the event in Hyde Park on September 11, along with other acts such as Will Young, James Blunt and Lenny Kravitz.
Barlow is replacing singer Lionel Richie, who was forced to pull out because of health issues.
Speaking about the performance, Barlow said: '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.'