Olympic stars Ellie Simmonds and Greg Rutherford will also star in two celebrity editions of the ITV quiz show in the new year.
Healy will team up with her rugby player husband Ben Foden on the January 1 episode, while comedian Omid Djalili will join explorer Grylls and Paralympic gold medal-winning swimmer Simmonds will partner singer Michael Ball.
Saturdays singer Healy said: "Ben and I are huge fans of the show and have watched it for years.
"When we were asked if we wanted to play for our chosen charities Marie Curie and Niamh's Next Step, we jumped at the opportunity. We can't wait to take part and hopefully raise lots of money for these great causes."
Rutherford will partner Dragon's Den star Duncan Bannatyne on the January 8 episode.
Bannatyne said: "I'm very, very nervous about coming on the show in front of a live audience. I just want to do my best and make as much as I can for Mary's Meals, which is a brilliant charity.
"It's an absolute privilege to be coming on with Greg, I think his gold medal win was brilliant - I just hope he knows more than I do!"
Rutherford added: "I'm both excited and nervous, and really looking forward to doing the show. I've seen it for years and years, but obviously taking part in the studio will be very different to watching at home!
"I'm determined to raise a lot of money for Sport England, and since Duncan's a very clever businessman, I think between us we'll hopefully do well."
Coronation Street pair Charlie Condou and Samia Ghadie will also appear on January 8, hoping to win money for The Terrence Higgins Trust and East Cheshire Hospice.
Jon Culshaw will also play for Starlight Grants with his co-star Debra Stephenson, who said: "I'm really excited about Who Wants To Be a Millionaire. I feel so privileged to have the chance of being able to raise a significant amount of money for my local children's hospice Julia's House, and therefore make a difference to the quality of life for children in Dorset whose life expectancy is under 18."
The Who Wants To Be a Millionaire? New Year's specials will air at 8pm on January 1 and January 8 on ITV1.