The former EastEnders star will play DCI Sacha Millard, the new boss of the Unsolved Crime and Open Case Squad (UCOS).
Outhwaite will appear in the final two episodes of the 10th series, following Redman's departure in episode eight.
The drama, which has been running since 2003, will return to BBC One later this year.
Outhwaite will appear alongside regular cast members Dennis Waterman and Denis Lawson in the popular crime series.
"I'm delighted to be joining the cast at such an exciting time," said the actress, who made a guest appearance in another role on the show's ninth series.
"I've been a huge fan of the show for years and can't wait to be working with the amazing cast and crew."
Only Fools and Horses star Nicholas Lyndhurst will also join the show when it begins its latest run.
Tamzin Outhwaite is to join the cast of BBC crime drama New Tricks following the departure of Amanda Redman.