Set in the leafy London suburb of the same name, "Hampstead" is inspired by true-life events and is being developed and produced by UK-based banner Ecosse Films, which was behind the 2009 John Lennon biopic "Nowhere Boy", said The Hollywood Reporter.
BAFTA-winning filmmaker Joel Hopkins (Last Chance Harvey) will direct from a screenplay by Oscar-nominated writer Robert Festiger (In the Bedroom), with Ecosse's Robert Bernstein and Douglas Rae producing.
The story sees Keaton, 69, playing American widow Emily Walters, who one day witnesses a man on Hampstead's famed Heath being attacked by a group of professional thugs.
The two form an unlikely alliance as the battle escalates.