The 43-year-old TV presenter had her big break hosting the ITV Saturday morning show with former child stars Ant McPartlin, 44, and Declan Donnelly, 45, from 1998 to 2002.
Deeley shared a snap of herself looking elegant in PVC trousers and a grey T-shirt, back on the famous sofa with the Britain’s Got Talent hosts.
She wrote: “SMTV reunion @antanddec ...... All those in favor say “I “.....”
Ant and Dec posted the same picture, captioned: “Excited?”, to which Deeley replied: “Yes?!?!”
They also shared a throwback snap from the original show.
Deeley wrote: “Those were the days ...”
The Birmingham-born presenter recently revealed of the reunion: “We shot a documentary and we talked about all different things, how we first got together how we first met each other, all the things that went wrong in the show.
“It was at a time of our lives when we didn't have many responsibilities really, it went quite badly at the beginning, but we turned it around, we found our feet. We found the chemistry between us.”
Watch: Cat Deeley cringes watching her SM:TV screen test
She added: “We went back to the studio where we did it. They rebuilt the set. We all cried. I walked in and [Ant and Dec] were doing their interview and Dec was crying and I was like 'Okay you're crying!' and then at the end of my interview I was like, ‘Waaah!'
“We all have a bit of a cry, we take the mickey out of each other, it's very, very, very cute.”
The reunion show will air at the end of the year.
The live Saturday morning magazine show featured comedy sketches, celebrity guests and interviews and cartoons.
Sketches included Chums, a parody of US sitcom Friends, which saw the three presenters all living in a flat together and featured a fictional romance between Deeley and Donnelly.
Before a segment when fan mail from viewers was read out, the presenters all performed a dance routine set to Please Mr. Postman by The Carpenters.
And a regular phone-in competition called “Wonky Donkey” challenged contestants to guess the rhyming couplet that described that week’s object.
After Deeley, McPartlin and Donnelly left the show it was hosted by Steps stars Ian “H” Watkins, 44, and Claire Richards, 42, between 2002 and 2003.
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McPartlin and Donnelly met when they in 1990 aged 13 while playing the characters PJ and Duncan in BBC children’s drama Byker Grove.
They launched a pop career as PJ & Duncan, releasing the Top 10 single Let's Get Ready To Rhumble in 1994, and subsequently built their TV presenting career from kids TV.
After SM:TV, Deeley went on to host Fame Academy and Stars In Their Eyes before moving to the US in 2006 and hosting reality show So You Think You Can Dance.
She recently returned to live in the UK with Irish comedian husband Patrick Kielty, 49, and their two sons Milo, four, and James, two.