The Girl on the Train actor split from the Friends star in February following their three-year marriage.
Speculation arose on Tuesday that the Leftovers star is spending his newly-found free time with the 25-year-old artist and model.
According to Star magazine, the 46-year-old is “obsessed” with the Canadian, who he has been “enjoying cosy dinner dates with”.
A source said: “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.”
The duo reportedly connected over the New York art scene and“hit it off right away.”
Last month, Theroux tagged Collins in an Instagram post shortly before his split with Aniston was announced.
The pair posed together in t-shirts crated by designer Adam Selman.
The news comes after speculation that Aniston has planned a “girls trip to Hawaii” in a bid to get over the actor.
A source told The Sun: "Jen loves a beach vacation - there's nothing she finds more relaxing than a trip to the beach, or lying by the pool.
"Normally she goes to Mexico, to the same place, but she doesn't want to go to Mexico for a while because it reminds her too much of Justin!
"There's talk of her going to Hawaii with ten friends, though - something really low key."