According to Star magazine the 46-year-old is "obsessed" with the pretty blonde and the pair have been enjoying "cosy dinner dates and exchanged flirty messages on social media."
"They've actually been pretty brazen," the pal revealed of their alleged blossoming romance.
"Justin and Petra have been carrying on for some time. He insists that they're just 'friends,' but he seems obsessed with her, and Jen isn't buying it," an insider explained to the publication.
"This is like history repeating itself… she's heartbroken and humiliated," they add.
The mag claims Justin and Petra met through the trendy New York art scene and "hit it off right away."
"Petra is totally Justin's type. She's hip, cutting-edge and has a cool downtown vibe. She's everything Jen isn't," the source continued.
Although the duo haven't been seen together just yet, tellingly Justin the tagged rising star in one of his Instagram snaps last month.
And in yet another interesting twist, Petra was once very close to Selena Gomez, who is in turn one of Jennifer Aniston's besties.
However, Selena has since un-followed Petra, who she once described as her "sister" on Instagram so as they say, where there's smoke...
Sel previously said she turns to Jen for "maternal advice" and loves the Friends beauty because "she's very cool and very sweet."
Petra isn't the first name to be linked to Justin.
In January last year when photos emerged of the actor deep in conversation with Aussie star Naomi Watts at The Unveiling: Class by Taryn Toomey exhibition in New York, insiders told the National Enquirer Jen was "upset" by the images.
WATCH: Jen and Justin in happier times. Post continues...
In the snaps Naomi, who split from long-term partner Liev Schreiber in 2016, and Justin can be seen sharing a warm embrace before enjoying a deep conversation over drinks.
Jennifer Aniston and Justin Theroux announced their divorce on February 15th after two years of marriage.
"Normally we would do this privately, but given that the gossip industry cannot resist an opportunity to speculate and invent, we wanted to convey the truth directly," Jennifer's publicist Stephen Huvane released in a statement.
"Whatever else is printed about us that is not directly from us, is someone else's fictional narrative. Above all, we are determined to maintain the deep respect and love that we have for one another."