He will stand in for Touker Suleyman, who was unable to film for part of series 17, which airs from Sunday (August 11) at 8pm.
Theo will appear later in the run, in a number of episodes alongside the current line-up of Dragons: Deborah Meaden, Peter Jones, Tej Lalvani and Sara Davies.
He originally joined the show in 2005 as part of the line-up in the second series. He took part for nine series between 2005 and 2012 but stood down in 2013 to focus on his retail businesses. (In July 2012, Theo bought added the 96-store homeware and housewares chain Robert Dyas.)
Commenting on his return, Theo said: “It was my Godfather moment… just as I thought I was out, they pull me back in – and it was a pleasure to breathe some Dragon fire into the Den again. When I received the call asking me to stand in for part of this upcoming season it didn’t take long to say yes – but it did feel like my Bobby Ewing moment, stepping out of the shower. The moment I sat in the chair, it felt like I had never been away. I did try to reclaim my middle chair from Deborah but she wasn’t having any of it.
“Being the first and only emeritus Dragon in the Den is an honour. I enjoyed relieving the current Dragons of some great investments along the way, and rekindling some old rivalry.”
Touker said: “I’m grateful that my good friend Theo was able to step in to the Den to keep my seat warm while I had some time off. I can’t wait to see him back in action and keeping the other Dragons on their toes. But don’t worry, I wasn’t gone for long!”