Moss, 41, had a cameo on the series in 2012 with fellow model David Gandy and fashion designer Stella McCartney, reported Daily Mirror.
"I can confirm Kate does have a part. It's not yet decided whether she'll have a speaking part but she'll definitely be in it," Saunder said.
"Absolutely Fabulous" saw characters Eddie Monsoon, played by the show's writer Saunders and Patsy Stone (Joanna Lumley) live in a boozy world of fashion and PR.
Saunders, 57, said Moss is her friend and it was a "no brainer" that she has joined the cast.
"I've only just got the green light so I'm tweaking the script and I'm trying not to over-egg it."
Saunders said filming will kick off next month on a location at London Fashion Week, where they will be recording a footage which will be weaved into the finished product.