The actress, who plays Detective Superintendent Sandra Pullman in the police drama, will film one more series before bowing out next summer.
> Denis Lawson replaces James Bolam on New Tricks
The show was renewed for two more series last year after regularly attracting over 8m viewers.
"I love New Tricks and I have loved playing Sandra - I've got some gritty plotlines - but I feel I have to try something new," Redman told The Mirror.
"And that is why I've decided I am going to film a further eight episodes and then I'll leave. The time's come. After a while, there is a danger of things getting stale."
Of her decision not to quit immediately, she explained: "It wouldn't have been fair on the audience. There is a loyalty to the fans. Our viewing figures are so high, it's only fair to do it properly.
"[The BBC] understand. They know I've been thinking about this for some time. I could have turned around and said I was leaving this year, but I didn't and they respect that."
Redman added that she "can't wait" to tackle new roles, but ruled out appearing in a period drama like Downton Abbey, instead preferring to star in "something new".
She was appointed an MBE in June for her work running a youth drama school, as well as her services to drama and charity.
New Tricks returns to BBC One on Monday, August 27 at 9pm.