Actress Lindsay Duncan will guest-star as the Time Lord's assistant in this year's second special.
Lindsay will play Adelaide, who is described as the Doctor's cleverest and most strong-minded companion yet.
She said: "I'm thrilled to be involved in Doctor Who. I've never done anything like this before."
There are four specials planned for 2009 with the first, entitled Planet of the Dead, being shown at Easter.
Russell T Davies and Phil Ford are joint writers of the second special.
Davies said: "Lindsay is an incredibly talented actress and I've been an admirer of her work for some time.
"We are delighted to announce that she will be joining the team and playing the Doctor's most strong-minded companion yet."
David Tennant will star in all four programmes, before 26-year-old Matt Smith takes over as the Time Lord in 2010.