And it seems she felt the late star's presence while she was filming.
Speaking as she promoted Blonde at Venice Film Festival, de Armas said: "I truly believe that she was very close to us. She was with us."
The actor, 34, said she thought Monroe was "happy".
"She would also throw things off the wall sometimes and get mad if she didn't like something," she told press.
"And this is true. These things happened. And being in the same places that she was, filming in her house, it was a very strong sensation. It was something in the air.
"And I think she was approving of what we were doing."
Blonde - which was written and directed by Andrew Dominik - paints a fictional portrait of the life of Monroe, documenting her life from being Norma Jeane Baker to becoming a screen icon.
“Andrew’s ambitions were very clear from the start - to present a version of Marilyn Monroe’s life through her lens,” de Armas previously told Netflix Queue.
"He wanted the world to experience what it actually felt like to not only be Marilyn, but also Norma Jeane. I found that to be the most daring, unapologetic, and feminist take on her story that I had ever seen.”
Read more: Ana de Armas nearly passed on Knives Out
The film debuted at the festival this week and received a 14-minute standing ovation.
Blonde will be on Netflix on 23 September.
Watch: Ana de Armas doesn't 'understand' why Blonde is rated NC-17