The five-time Paralympic champion and broadcaster said she is "literally bursting with excitement" but also "absolutely petrified" about taking to the famous dance floor.
The 27-year-old, who at the age of 13 was the youngest British athlete at the 2008 Paralympics in Beijing, said on Twitter: "Oh my gosh! I’m literally bursting with excitement!"
She adds: "It’s been soooo hard keeping this secret as I’ve wanted to tell everyone. I’m going on Strictly Come Dancing!“
"I’m a tad nervous too, well more than a tad… I’m absolutely petrified!
"To be asked to be part of the Strictly family is just so exciting and I cannot wait, it’s going to be so much fun.
"I’m counting down the minutes until it starts; learning to dance, meeting my partner and everyone else involved in the show. Plus wearing the sequins and all that, it’s going to be a blast."
Simmonds continues: "I’ve been watching Strictly from as far back as I can remember; it’s a traditional lead up to Christmas with my family and it’s a genuine privilege to be part of it."
The swimmer joins the previously confirmed contestants, comedian and actress Jayde Adams, Coronation Street actors Will Mellor and Kym Marsh, Loose Women star Kaye Adams and Radio 2’s Richie Anderson.
The last series of Strictly was won by Rose Ayling-Ellis with professional partner Giovanni Pernice.
The forthcoming 20th series will see audiences return to the studio, with the launch show scheduled to be recorded on September 7.