The hit series follows Detective Superintendent Sandra Pullman (Amanda Redman) and her team of retired ex-cops as they solve the Met's unsolved crimes.
Lawson will replace James Bolam, who announced his departure from the show in September.
His character Steve McAllister is described as "a bundle of energetic optimism" and "the antithesis" of Alun Armstrong's Brian Lane.
"I'm delighted to be joining Amanda, Alun and Dennis [Waterman] on New Tricks," said Lawson. "I've long admired the show and love its ability to be warm, humorous and gritty simultaneously. I'm really looking forward to being part of it."
Executive Producer Richard Burrell added: "Casting a new member of the UCOS team was always going to be an exciting challenge and we are absolutely thrilled that the brilliant Denis Lawson is joining us.
"Denis brings warmth, wit and charm to the character Steve McAllister and is a very welcome addition to the New Tricks family."
Lawson's recent television credits include Holby City, Criminal Justice and Marchlands. He is also known for playing Wedge in the original Star Wars film trilogy.
The ninth 10-part series of New Tricks is currently filming at Pinewood Studios and on location in London. The series will return to BBC One later this year.
> New Tricks renewed for two more series by BBC One