The R&B star, whose single 'Down' is currently at number two in the US, admitted that he had grown close to Buchanan since working together earlier this year.
"We bumped into each so many times at different events and sometimes you get drawn to each other. Keisha's a real good friend," he told Alex and Lucy on Key 103's IN:Demand.
"We got in the studio and did a track called 'Fire Away', which is a beautiful song. Keisha is a great songrwiter and we both wrote it together.
"Hopefully you'll be hearing that by the end of the year. She's got an amazing voice."
Sean added that he was shocked when he heard that Buchanan had been ousted from Sugababes last month.
"I talked to her when everything was happening. She called me up when she was in Los Angeles and I was in New York. It was horrible, she was by herself there. It wasn't nice," he said.
"She was really upset and stuff. I said that the media is going to make up a lot of crap because it's more interesting than saying nice things.
"I can vouch for her, she's a lovely girl. She's been there from the beginning and people don't remember that. She was the original Sugababe and I've got a lot of love for her."
Jay Sean releases 'Down' in the UK on October 26.