In his new memoir The Making of 'Some Like It Hot', the actor describes his liasons with the blonde bombshell in explicit detail, and even claims the baby she conceived during their 1958 affair was his.
Curtis goes on to recall the day the star announced her pregnancy in October 1958 during a heated exchange with her playwright husband Arthur Miller, who refused to believe the baby was Curtis' child.
And although Curtis' then-wife Janet Leigh was also pregnant with their second daughter, Jamie Lee, the 84 year old insists Monroe was intent on mothering the child she eventually miscarried.
The actor also reveals their first tryst actually came in 1950, when Monroe was first launching her career - and he insists he'll never forget their romps.
He writes, "When I was in bed with Marilyn, I was never sure, before, during or after, where her mind was. She was an actress. She could play a part. She could give the part what she thought a man wanted. I never asked for more. What I experienced with her was unforgettable.
Curtis' book is set for release on 28 September (09).