This year is the 40th anniversary of Doctor Who - which was on TV from 1963 until 1989.
This is the first major change in the lead actor since Paul McGann took over the role of the Doctor in a 1996 TV movie.
Although Richard E Grant did have a taste of playing The Doctor in a Comic Relief special in 1999.
He's appeared in films like The Little Vampire and Spiceworld and starred in The Scarlet Pimpernel on TV.
The Doctor is a Time Lord who fights evil beings across time and space - his most famous enemies are the Daleks.
The Doctor travels in a ship called the Tardis that's shaped like a police phone box and is bigger on the inside than it is on the outside.