The 31-year-old actor announced the news on The Howard Stern Show Tuesday.
"We're marrying, it's happening," Stewart said.
She also revealed that Meyer did the proposing.
"I wanted to be proposed to so I think I very distinctly carved out what I wanted and she nailed it," Stewart said.
The two started dating in 2019 after they met at a mutual friend's party and bonded over having grown up in Los Angeles.
A week ago, Meyer posted a photo on Instagram of them lying on the floor in each other's arms with the caption, "Awful proud of this hard-working princess."
Stewart is currently promoting the Princess Diana film "Spencer," which comes out in theaters on Friday.
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Note: The video in the featured media player above is from a previous report.