Deeley, who worked on the BBC show with Kielty in 2002 and 2003, described him as "a really old friend" during an appearance on talkshow Live! with Kelly.
The So You Think You Can Dance host said: "I am no longer a single lady. I am dating a really old friend of mine. I have known him for 10 years.
"I don't know [why it took so long]. Ladies just like to take their time. Now I have finally seen the light."
The pair were first romantically linked in February of this year, and comedian and broadcaster Kielty was reportedly overheard confirming their relationship shortly afterwards.
They appeared at a red carpet event as a couple for the first time at last week's Glamour Woman of the Year Awards.
Deeley was recently confirmed as host of Fox's new celebrity dating show The Choice.
Blending elements of The Voice's spinning chairs format with speed dating, The Choice premieres on Thursday, June 7.