Karen, who played the Time Lord’s assistant Amy Pond, revealed her exterminated bonce at a Marvel comic convention in San Diego, US.
The telly star shaved off her hair for a role in Marvel film Guardians of the Galaxy.
She told the crowd: ‘It’s liberating. I think everyone here should shave their heads.’
The 25-year-old actress waited until she was on the film's panel to announce the makeover, first appearing at the convention on Friday wearing a red wig.
Due out in 2014, the movie also stars Chris Pratt, Michael Rooker, Benico Del Toro and Zoe Saldana.
Karen has just finished promoting her new rom-com Not Another Happy Ending.
She appeared at the movie's world premiere in Edinburgh sporting flowing red locks down to her waist.
