The shocking split came as a surprise, given how happy 47-year-old Watts and 48-year-old Schreiber have appeared to be for more than a decade. The two had been dating since 2005, when they met at the annual MET Gala and hit it off. Schreiber told Esquire in June that their romance began when Watts invited him out dancing after the Gala, but she was surrounded by actors Benicio Del Toro and Sean Penn.
"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," he admitted.
Though it turns out he had no reason to sweat -- Watts chased him down and gave him her phone number, making the first move.
"He got up to say he was leaving and I was like, 'Oh s**t.' Probably another couple years are going to pass by," the Australian actress told Allure in 2013 about the fateful moment. "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?'"
"It was a lot more ballsy than I would ever normally be," she added.
The rest, of course, is history. Though the two never married, they are parents to their two sons -- 9-year-old Alexander "Sasha" Pete and 7-year-old Samuel "Sammy" Kai -- and often gushed about spending family time together.
Over the summer, the adorable foursome spent quality time in New York City, where they're based, checking out Hamilton on Broadway, and going for adventures in Central Park.
"My tribe," Watts sweetly captioned a family selfie earlier this year.
When ET spoke to Watts all the way back in January 2009 at a NYC screening of Schreiber's Defiance, she said she tagged along on set in the "freezing woods of Lithuania" just to spend time together with him, and a then newborn Sasha.
"We just had our first baby and we all wanted to stay together as a family, and trekked along," Watts explained. "It was an adventure for all of us."
And when ET was on the set of his Showtime hit, Ray Donovan, in June 2013, Schreiber admitted that filming in Los Angeles and away from his family was the toughest part of the gig.
"I think that's been the hardest part for me. It's been the hardest part not having the boys around all the time," Schreiber said. "What we've got to figure out is how to balance the work and family stuff, you know. I need to be around them to do this, so, we'll work it out."
Watts also admitted to ET that she struggled with balancing work and family when we spoke to her in March while she was promoting Divergent.
"Yeah, I mean there are days where you feel like you're supermom and doing really well and got it handled, and then there are days where you're like, 'Ah! I can't do this!'" she said candidly. "It's like any working mom, I think. You know, it's a balance."
But when it came to putting in effort in the romantic side of the relationship, Watts and Schreiber appeared to be doing perfectly fine, going on sweet date nights and supporting one another professionally. The two worked on their upcoming film, The Bleeder, together, and happily promoted the film earlier this month at high-profile events including the Toronto Film Festival and the Venice Film Festival.
Watts also proudly shared her support for Schreiber when the actor was part of the 2015 Oscar-winning film Spotlight, accompanying him to every awards show earlier this year.
When ET caught up with the two at the Screen Actors Guild Awards in January 2016, Schreiber said it was "date night" for the parents. As for how they made their longtime relationship work, he quipped, "We really like each other."
"It helps to like the person you're with," Watts echoed.
The spark between the couple appeared to still be going strong earlier this month, when Liev made it no secret that he was smitten with Watts' stunning good looks. The actor Instagrammed a pic of her donning a sexy thigh-high boots, writing, "Ay dios mio..."
In a joint statement to ET on Monday, Watts and Schreiber said they're now looking forward to "a new phase" in their relationship, and joint-parenting.
"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," the statement read. "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."
Watch the video below for more on the shocking breakup.