The "Harry Potter" actress and R&B star, who had been rumoured to be taking part in the reality show, confirmed their participation with an appearance on "Good Morning America" on Wednesday, September 12.
The Irish star, who portrayed Luna Lovegood in the franchise, will show off her dancing skills alongside professional Keo Motsepe, while Tinashe will work with Brandon Armstrong.
Evanna shared their official "DWTS" picture on Instagram, and added, "See these faces? These are the faces of WINNERS. Right, @keo_motsepe ?! I could not be more thrilled to be part of this show and to have the best partner! Let's go season 27!"
Tinashe did the same, alongside the caption, "The rumors are true! Catch me on this season of Dancing with the Stars! #DWTS."
The celebrities join "The Facts of Life" star Nancy McKeon and Disney star Milo Manheim, who had previously been announced.
Other contestants for season 27 include former "Dukes of Hazzard" actor John Schneider, who hit headlines earlier this year for serving five hours of his three-day prison sentence in June for failing to pay his estranged wife spousal support.
He is joined by retired gymnast Mary Lou Retton, the first U.S. woman to win the individual all-around Olympic gold medal, and her fellow sport stars, former American footballer DeMarcus Ware and alpine skier Danelle Umstead, a member of the U.S. Paralympics team.
The line-up is rounded out by "Fuller House" actor Juan Pablo Di Pace, comedian Nikki Glaser, radio personality Bobby Bones, model Alexis Ren, and Joe 'Grocery Store Joe' Amabile from reality shows "The Bachelorette" and "Bachelor in Paradise".
The new season airs in the U.S. from September 24.