The Silent Witness actress will play a love rival to French's character in the Sky1 show.
Others stars joining the cast include Game of Thrones' Iain Glen and seasoned actress Sheila Hancock.
The four-part drama, written by Mr Selfridge's Dan Sefton, is described as an "honest and compelling story of love, sex, lies and betrayal".
Fox said she was "absolutely delighted" to be part of Delicious.
"As a starter it's heaven to be working with Dawn French and Iain Glen again," she said.
"For main course - an absolute treat to be working on such great scripts. For the perfect recipe - a brilliant crew, cast and production team. I can't wait to tuck in."
Glen said: "The script for Delicious was irresistible.
"Funny, poignant and unpredictable. I can't wait to dig my teeth into this feast of a drama."
French plays the culinary genius Gina, who was once married to Glen's character Leo Vincent, a successful entrepreneur, chef and hotel owner.
Fox's character Sam, Leo's new romantic partner, confides in Gina over her suspicions that Leo is having an affair - only to discover that Gina herself has become the "other woman".
French and Fox last starred opposite each other in James Corden's BBC Two comedy The Wrong Mans.
Delicious is expected to be broadcast later this year or early in 2017.
Follow us on Twitter @BBCNewsEnts, on Instagram at bbcnewsents, or email entertainment.news@bbc.co.uk.
Emilia Fox is to star alongside Dawn French in the forthcoming TV drama series Delicious.