“I can’t believe our little mate has gone and it’s very real and very raw,” he added.
“He was the best.”
Fellow Radio 1 presenter Charlie Hedges took over presenting duties on the breakfast show on Tuesday as James took some time off.
Radio 1 listeners were among those to offer their condolences, with one texting in: “Greg if you’re listening, we all love you mate.”
James and Mackie adopted Barney from Battersea Dogs and Cats Home in 2019.
Barney had previously visited Professor Noel Fitzpatrick, external, who starred in The Supervet, on a number of occasions, including to treat his arthritic joints.
Earlier this year, James took on a tandem cycling challenge for Comic Relief, which saw him travel 1,000km (630-mile) through England, Wales and Scotland.
To help motivate him through the challenge, one of the videos he received , externalwas of his dog Barney frantically barking at foxes in their garden.
