The 31 year-old broke the news on Tuesday as she was doing press for her upcoming film portraying Princess Diana.
Stewart, who has been with Meyer for two years, told Stern on SeriousXM that she had accepted her partner's proposal.
"I wanted to be proposed to, so I think I very distinctly carved out what I wanted and she nailed it. We're marrying, it's happening."
Meyer is a screenwriter, who co-wrote and executive-produced the Netflix film 'XOXO'. She has also wrote episodes for the sci-fi show 'Miss 2059'.
The couple met six years prior to them getting together, but reconnected in 2019.
"She rocked up at a friend's birthday and I was like, ‘Where have you been and how have I not known you?"
Stewart famously was in a relationship with her 'Twilight' co-star Robert Pattinson for four years. The couple broke up after Stewart's affair with 'Snowhite and the Huntsman' director Rupert Sanders went public.