The actress has played the Agatha Christie heroine in 12 films for ITV since 2003, two of which have yet to be aired.
The feature-length adaptations proved a hit with viewers, attracting 10 million at their peak.
McEwan explained that she had fond memories of playing the character, saying: "Of course, it is terribly disappointing to have to pass the baton on, but it has been a marvelous experience to inhabit this role for the last few years, and I am sure that my successor, whomever she may be, will thoroughly enjoy her time with the production team - and with the extraordinarily talented group of writers and actors that they come up with for each series.
"It's been an absolute pleasure to work on Miss Marple since 2003, and I leave with fond memories."
Mathew Prichard, the grandson of Agatha Christie and the chairman of Agatha Christie Ltd, described McEwan as a "wonderful Miss Marple", saying: "I think she brought a humanity and playfulness to the role that worked beautifully in the context of the new approach we adopted for this ITV series.
"We will miss her - but are looking forward to seeing what new aspects of Jane Marple's character our new Miss Marple might be able to explore."
ITV and Marple rights owner Chorion will begin the search for an actress to take over the role next week.
The character was previously played by Gracie Fields, Margaret Rutherford, Angela Lansbury, Helen Hayes and Joan Hickson.