The 'A Most Violent Year' star said she is fantastic in her three year old relationship, reported People magazine.
"I'm not quite sure of what I want in my life, and who knows if marriage is a part of it. So to me, marriage is not an important thing," she said.
The star - who appeared in four movies last year, including the sci-fi blockbuster Interstellar - is earning critical raves as a steely wife in the crime drama 'A Most Violent Year'.
"That was a Chastain invention. I had no idea people would be coming up to me and flicking a cigarette and quoting my line, 'This was very disrespectful.' It's very cool," she added.