The two dated in the 1970s as Keaton starred in many of Allen's most popular films, including Love And Death, Annie Hall and Manhattan.
Keaton has revealed in her upcoming memoir Then Again that she fell "in love" with Allen years before she ever met him.
"How could I not [love him]? I was in love with him before I knew him. He was Woody Allen. Our entire family used to gather around the TV set and watch him on Johnny Carson. He was so hip, with his thick glasses and cool suits," she wrote in an excerpt published in Vogue.
"It was [Woody's] manner that got me, his way of gesturing, his hands, his coughing and looking down in a self-deprecating way while he told jokes like 'I couldn't get a date for New Year's Eve so I went home and I jumped naked into a vat of Roosevelt dimes'. He was even better-looking in real life. He had a great body, and he was physically very graceful."
Keaton also teased that her and Allen's relationship lasted so long because they accepted each other's quirks.
"We shared a love of torturing each other with our failures. His insights into my character were dead-on and hilarious. This bond remains the core of our friendship and, for me, love," the star wrote.
Allen and Keaton broke off their romance and professional partnership in the late 1970s, but remain close friends to this day.
Keaton made a cameo in Allen's 1987 film Radio Days and later reunited on-screen with the comic six years later in Manhattan Murder Mystery.
Allen admitted in March that he would love to work with Keaton again, but will wait until the right project comes along.
Keaton's autobiography Then Again is released on November 15.
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Watch a clip of Diane Keaton and Woody Allen in Sleeper below: