The announcement will come as part of a "Doctor Who Confidential" program (or "programme") airing on BBC One (or "The Beeb") at 5:35 p.m. GMT.
Tennant replaced Christopher Eccleston who took over the role in 2005. Tennant is the 10th time-travelling doctor and will stick with the franchise through four special episodes that will film this year and will probably air through at least early 2010 in the U.K.
The new Doctor will begin shooting their first series this summer with a British premiere in 2010.
But which actor will step into the Tardis and go head-to-head with Daleks and whatnot?
The scuttlebutt suggests that the next Doctor Who will either be an actor familiar to American actresses or he won't! And that he either will be a white dude, like the 10 previous Doctors, or he won't!
More familiar and conventional candidates bandied about have included "Viva Blackpool" (and "Basic Instinct 2") star David Morrissey, "Life on Mars" star Jon Simm or James Nesbitt, who worked with current "Doctor Who" executive producer Steven Moffat on "Jekyll."
But BBC reports say that another early favorite was "Survivors" star Paterson Joseph, who also appeared on "Jekyll" and would become the first black Doctor Who.
Or will we see the first female doctor? A combination of rumor mongering and wishful thinking has linked "Ab Fab" star Jennifer Saunders, Oscar winner Catherine Zeta Jones and former Doctor Who assistant Billie Piper to the role.
My own favorite Doctor Who? Clearly Tom Baker, for the scarf and the 'fro.
When the official announcement is made on Saturday, HitFix will have the Doctor news.