Mystery solved.

A month after Rachel McAdams and her True Detective costar Taylor Kitsch dined at West Hollywood’s Osteria Mozza on June 2, multiple sources confirm they are dating.

“It hasn’t been long,” says an insider. “But it’s serious.”

The Canadians, who recently wrapped the HBO drama’s second season, have been “friends for years,” adds a second McAdams source. “They grew closer on set.”

McAdams, 36, once told Glamour that work isn’t a convenient place to find love. (Who can forget her IRL romance with The Notebook costar Ryan Gosling?!)

“A set may seem like a good place for romance, but I don’t think it’s very conducive; it’s too distracting,” she says. “Every relationship I’ve had with a coworker comes after the fact.”

But McAdams couldn’t resist Kitsch’s charm — or Jake Gyllenhaal’s.

In April, “she definitely dated Jake Gyllenhaal while filming Southpaw,” the insider continues.

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Kitsch, 34, who recently broke up with fitness instructor Jennifer Welch, has had no problem sweeping McAdams off of her feet.

“I don’t know if it’s full-blown love yet, but they talk constantly,” another source confirms. “They’re really into each other. She talks about him all the time!”

A rep for Kitsch did not return our request for comment.

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