Doctor Who's new assistant will be played by a familiar face to fans of the show, Freema Agyeman.
Freema appeared in the most recent episode of the show, Army of Ghosts, where she played Adeola.
But Adeola died in that episode, so Freema will play a brand new character, called Martha Jones.
Executive producer/writer Russell T Davies said: "We're delighted to have chosen her to join the Doctor for more adventures in time and space."
He added: "The first scripts have been written, and Martha's a brilliant addition to the Tardis crew, the perfect foil for the Doctor.
Freema played Adeola in series 2's Army of Ghosts
"Martha won't be featured in this year's Christmas special; we've got another surprise in store for that."
Freema Agyeman says: "I've been keeping this secret from my friends for months - it's been driving me mad!
"Auditioning with David in secret down in Cardiff was unbelievable, but I never in my wildest dreams thought I'd actually become the new companion."