The film, which stars Ethan Hawke and Lena Headey as parents in a society where all crime is legal once a year, shocked industry insiders who expected "The Purge" to vie for the third box office slot, according to USA Today.
But despite mediocre reviews, the horror flick beat out a far more expensive film, "The Internship," which cost $58 million to make.
"We didn't expect anything near this result," Nikki Rocco, president of Universal's domestic distribution told the Hollywood Reporter over the weekend. "The social media campaign really paid off, as well as the traditional campaign."
Blumhouse, producer Jason Blum's company, confirmed the news over Twitter: