The Hollywood stars have been together for more than seven years and have two sons - five-year-old Alexander and Samuel, four - but Naomi revealed the pair don't feel the need to make it official.
She told Manhattan magazine: "Marriage? It feels like, 'Why fix it if it ain't broke?'."
But she revealed she works hard to balance her film career with being a mum.
"It's very rare that I miss a bedtime," said the 44-year-old.
Naomi admitted playing Princess Diana was difficult for her.
She said: "It was the hardest thing I've done because of the pressure of everyone's beliefs about who she was, and the fact that it was just really hard to claim her as my own, since everybody else feels they know her."