Ryan, who also had a recurring role on the first series of BBC One's hit fantasy drama 'Merlin', will play a mysterious character called Lady Christina de Souza.
She accompanies David Tennant's Doctor on a dangerous bus trip in 'Planet of the Dead', the first of four 'Doctor Who' specials which will air this year.
Comedian Lee Evans will also make a cameo appearance as a character called Malcolm, whose life becomes connected to the Doctor's in unusual circumstances.
There has been speculation that Ryan could be cast as the Doctor's new assistant when Tennant's replacement Matt Smith takes on the role next year.
Lily Allen and Rachel Stevens have also been tipped to play the part.
Catherine Tate's character Donna Noble first appeared in a 'Doctor Who' Christmas special 15 months before she returned as the Doctor's companion.
Ryan, who previously auditioned to replace Billy Piper, but lost out to Freema Agyeman, said she was "very excited to be joining David Tenant and the 'Doctor Who' team".
Russell T Davies, executive producer and writer on the show, said: "The script is being kept strictly under wraps -- however we can reveal that lady Christina is a woman with a mysterious past who's going to have a huge impact on the Doctor."
Filming on the episode, which will air in April, began in Wales this week.
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