The US singer went out with the Mad Men actress for over two years before they decided to split in 2006.
"We were madly in love. It was definitely my longest relationship," he told Details.
Josh, who releases his new album All That Echoes on February 4, admitted that he would like to find love again.
"I'd love to get into another serious relationship. I am a real romantic at heart," he added.
"We're very good friends. We met before her first album was even released, and we hit it off because we're both goofballs," he said.
When asked about it possibly being a bit more serious, Josh admitted: "We might have skated on the line of dating."