Ethan Slater's wife 'blindsided' by his relationship with Ariana Grande

She didn’t see it coming.

Ariana Grande is dating her married “Wicked” co-star Ethan Slater, and his wife, Lilly Jay, was “completely blindsided” by the relationship, according to sources familiar with the situation.

“It’s horrible. They were high school sweethearts. They have a baby! She’s a wreck,” a source tells Page Six exclusively.

Slater and Jay have been married since 2018 and celebrated their 10-year anniversary together as a couple last November.

He and Grande have been filming “Wicked” in London for the past year, and sources believe she’s the reason Slater and Jay “suddenly break up now when nothing appeared wrong between them,” the source says.

Slater, 31, even gushed over his “wonderful” wife in May in an Instagram post for Mother’s Day.

“Happy first Mother’s Day to the most loving caring and wonderful mom/person in the world,” he wrote.

The post included a photo of their son’s hands.

Slater has since gone private on the social media app.

Production sources tell us Jay, 32, and their baby visited the “Wicked” set — and we hear Grande, 30, has spent time with the couple together.

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Meanwhile, we reported earlier this week that Grande and her real estate agent husband, Dalton Gomez, separated in January after two years of marriage but have remained “incredibly amicable.”

“They came to the decision together. They were having problems before January, but they want to remain best friends,” an insider told us.

Grande and Slater have not publicly confirmed their new relationship, and reps have not responded to multiple requests for comment.

Jay did not get back to us.