The Welsh actor will play a pivotal figure from Holmes's past, Entertainment Weekly reports.
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© CBS
Rees is currently only signed for a single appearance but his character may recur later in the first season of the Conan Doyle adaptation, which stars Jonny Lee Miller as Holmes and Lucy Liu as his partner Joan Watson.
Elementary exec Robert Doherty previously hinted that the show is "likely" to introduce the detective's estranged father.
The 68-year-old Rees is known for playing Lord John Marbury on The West Wing and for his role as Robin Colcord on sitcom Cheers. He also recently played a recurring role on Syfy's Warehouse 13.
Elementary premieres on Thursday, September 27 on CBS. In the UK, Sky Living will air the drama.
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Watch a promo for Elementary below:
