Julia Roberts discovered a centuries-old marital affair involving her great-great-grandfather on PBS’ Finding Your Roots with Henry Louis Gates Jr.

Julia Roberts is not who she thought she was.

The Oscar-winner appeared on PBS’ Finding Your Roots with Henry Louis Gates, Jr. on Wednesday, and was left speechless by a hidden centuries-old secret.

After researching Roberts’ family tree, Gates Jr. discovered that the man who was supposedly her great-great-grandfather, Willis Roberts, actually died more than a decade before her great-grandfather was born.

While her great-great-grandmother Rhoda Suttle Roberts had been married to Willis since the 1850s, his death in 1864 left a startling gap.

“Oh, wait — but am I not a Roberts?” the Ticket to Paradise star asked as Gates Jr. walked her through his findings.

Gates Jr. then explained that public databases instead found “a cluster of matches” that traced Roberts, 55, to a man called Henry McDonald Mitchell Jr.

“So we’re Mitchell’s?” Roberts asked.

“You’re Julia Mitchell,” Gates Jr. confirmed as she laughed. “You are not a Roberts, biologically.”

To make matters even more startling, the historian added that Mitchell had been married at the time of his affair with Roberts’ great-great-grandmother Rhoda and had six of his own children.

Through painstaking research of census data, he also discovered that Mitchell’s mother, a widow, lived “just four households” away from Rhoda.

“Wow,” is all Roberts could manage to say as Gates Jr. illuminated her true family history until he asked her how she felt about her newfound ancestry.

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“On the one hand, I — truly — my mind is blown,” Roberts said. “And it is fascinating. And on the other hand, there’s part of me that, when I’m calmer, can still wrap my arms around the idea that my family is my family. And I do prefer the name Roberts!”

“This was a very unexpected turn,” she added as Gates Jr. wrapped up his findings, telling her that Mitchell Jr. seemingly disappeared from the paper trail in the late 1800s. His and Roberts’ newfound ancestors, however, can be traced all the way back to 18th-century colonial Virginia.

Julia Roberts. ANCESTRY/YOUTUBE

Roberts’ family scandal is the latest revelation on the show. On Jan. 4 Edward Norton featured on the PBS show’s premiere episode and was told that 17th-century Powhatan woman Pocahontas is his 12th great-grandmother.

Gates. Jr. revealed that Norton’s ancestry — which includes a Civil War soldier who wrote to Abraham Lincoln and a late 19th-century pro-union labor activist — can be traced directly back to the earliest days of colonial America in Virginia, when Pocahontas married English settler John Rolfe in 1614.

The historian also revealed that Norton and Roberts share a similar enough DNA sequence to determine they share a common ancestor — news that left both actors laughing with surprise. “How come I didn’t get the teeth and the smile?” Norton joked.