The announcement will be made during half time of the FA Cup semi-final Match of the Day Live: Everton versus Manchester United at around 18:00 BST.
The new companion will star alongside Peter Capaldi's Time Lord in the 10th new season of Doctor Who.
They will replace Jenna Coleman, who played Clara Oswald, who left the show in 2015.
Coleman joined Doctor Who in 2012, and starred alongside two Doctors, Matt Smith and Peter Capaldi.
She asked to be written out and left to take on the role of Queen Victoria in a major ITV drama series.
Filming for the next series of Doctor Who will start this year but will air in 2017.
In January, it was announced the head writer and executive producer of Doctor Who, Steven Moffat, was stepping down from the show.
The next series will be his last, after which he will be replaced by Broadchurch writer Chris Chibnall.
The new Doctor Who companion will be revealed live on BBC One on Saturday night.