The two former celebrity contestants will take to the dancefloor one last time to raise money as part of the BBC One night of fundraising.
Widdecombe will partner with Anton Du Beke and Grant will reunite with Flavia Cacace to provide "an unmissable performance".
"I'm thrilled to be back on Children in Need as I was in the first one ever!" said Grant.
"And to be dancing with Flavia once more is a Strictly dream come true. A fantastic cause doing something I love! Everyone's a winner!"
Children in Need takes place on Friday, November 16.
Sir Terry Wogan, Fearne Cotton, Tess Daly and new Radio 1 breakfast show host Nick Grimshaw will host the evening.
One Direction, Little Mix, Susan Boyle, Leona Lewis, The Script, Paloma Faith, Il Divo and Tim Minchin will all perform during the evening.
Meanwhile, Girls Aloud will reunite live in the studio for the exclusive first TV performance of the official Children in Need 2012 single 'Something New'.
The evening will also include exclusive previews of the Doctor Who and Call the Midwife Christmas specials.