She will replace Amanda Redman as head of a team of retired ex-coppers reinvestigating the unsolved crimes.
Redman’s character Sandra Pullman will bow out in the eighth episode of the new series, currently filming in London, with Outhwaite’s DCI Sacha Millard taking over for the final two episodes.
The last series saw the departure of James Bolam who was replaced by Lawson. Redman’s fellow original cast member Alun Armstrong has also left the show.
Outhwaite will be joined by series stalwart Dennis Waterman and newcomers Denis Lawson and Nicholas Lyndhurst.
The actress, who previously appeared in EastEnders, The Fixer and Hotel Babylon, said was was “delighted to be joining the cast at such an exciting time”
“I’ve been a huge fan of the show for years – and can’t wait to be working with the amazing cast and crew.”
Series Executive Producer, Richard Burrell added: “The combination of Tamzin, Dennis Waterman, Denis Lawson and Nicholas Lyndhurst ensures that the audience will continue to enjoy the antics of UCOS as they enter their second decade of pursuing justice for the victims of unsolved crimes in their own inimitable way.”
The series returns later this year with a two-part special shot in Gibraltar.