The couple released a joint statement on Monday confirming the sad news, which comes just days after another apparently solid Hollywood couple - Brad Pitt and Angelina Jolie - split.
Naomi and Liev's statement reads: "Over the past few months we've come to the conclusion that the best way forward for us as a family is to separate as a couple.
"It is with great love, respect and friendship in our hearts that we look forward to raising our children together and exploring this new phase of our relationship. While we appreciate your curiosity and support, we ask the press to be mindful of our children and respect their right to privacy."
Reports suggest the two stars have been living apart for some time, but they are still actively raising their sons, Alexander, nine, and seven-year-old Samuel. They were last spotted together with their kids in New York City on 13 September (16).
Liev and Naomi never wed, but they had been together since 2005.
Schreiber recently recalled his first date with the King Kong actress, after they were introduced in at the Metropolitan Museum of Art's Costume Institute Gala, and Naomi suggested they hit a club together.
Liev told Esquire magazine he felt "very embarrassed" when he arrived at the nightspot to find his soon-to-be girlfriend alongside her Hollywood pals Sean Penn and Benicio del Toro.
"I went down to this club that she told me to meet her at, and of course she was there with Sean and Benicio, and I’m kind of standing around like a bump on a log, waiting for my turn," he said. "I think I was nervous, and I felt very embarrassed being there, and self-conscious, because all these movie stars were hovering, and I felt... awkward and out of my element. And I wasn’t going to make a good impression.
"I said, 'I’m sorry, but I gotta go home'. She chased me outside and said, 'Don’t you want my digits?'"
And Naomi recalled that moment in an Allure magazine interview, revealing, "Normally I want a guy to make the first move, and he wasn't (going to)... I was wildly attracted to him and his work, so I said something cheeky as he was about to walk out. I said, 'Don’t you want my digits?' Because I couldn't possibly say, 'Would you like my phone number?'
"It made him laugh. He texted me five minutes later saying, 'Would you like to go for a drink now?' And I said, 'No, I'll meet you for breakfast tomorrow'."
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