Burton recently submitted the court docs and listed March 1 as the date of separation, citing "irreconcilable differences" as the reason behind their split following her baby bombshell.
As RadarOnline.com reported, Gustin announced she was pregnant with her fourth child earlier this year.
Soon after receiving congratulatory messages, Burton set the record straight about the status of their relationship and her child being fathered by another man.
"I wanted to clear something up. Sheree and I are separated," he wrote. "She recently announced that she's expecting her 4th child. The child is not mine."
"We are still co-parenting our three beautiful kids," he added. "We would appreciate privacy at this time. Much luv, Steve."
The now-exes have been married since January 1999 and are parents to Makena, 18, Jack, 16, and Brooklyn, 7.
Burton is seeking joint legal and physical custody of their kids.
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Burton, also known for his appearances on The Young and the Restless and Days of our Lives, has also asked the court to terminate either party's ability to get spousal support once they legally part ways.
Back in December, there were no signs of trouble in paradise. At the time, they were posing for Christmas photos with their kids on her Instagram. Before that, she shared a photo of Burton spending time with their daughter.
Also, there's been a lot of professional changes for Burton in recent months.
Just last year, the actor announced he was let go from his role as Jason Morgan in General Hospital because he didn't want to get vaccinated for COVID-19.
"Look, the circumstance [of my GH departure] was tough and you know, that's just the way it is," he told Deadline in a new interview, revealing he was excited for his next opportunity. "I'll always be grateful for my time there. It was my family for so long. I grew up there and the fans have always been awesome to me. But again, some doors shut and other doors open and here we are in Beyond Salem. Wow."