Randolph, 25, filed on Friday in Los Angeles, according to court documents obtained by TMZ.
Reps for Randolph and Underwood did not immediately respond to PEOPLE's request for comment.
News of Randolph taking legal action against Underwood, 28, comes after the former couple called it quits in May. The breakup came two months after Randolph helped nurse Underwood back to health after he contracted the novel coronavirus (COVID-19). The pair met on Underwood's season of The Bachelor, which aired last year.
During their split announcement, the pair showed each other love on Instagram, commenting on their respective breakup posts with heart emojis on Instagram.
A source told PEOPLE at the time that the two "just weren't on the same page."
"Colton really wanted to settle down and get married — he's been ready for a long time," the source said. "And Cassie just wasn't ready. They both knew it wasn't going to work."
A second source told PEOPLE the breakup was "amicable," and that Randolph and Underwood wanted to remain friends.
Things took a turn in July after Randolph made an appearance on The Bachelor: The Greatest Seasons Ever! During the episode, she was reluctant to address their split and later called out the show for how the interview was "edited" to focus on her quotes about Underwood.
Without calling Randolph out directly, Underwood then noted on his Instagram Story that while the two had agreed to keep their split private, "obviously a lot changed this week." Randolph fired back at her ex with a lengthy note on her own Instagram Story, insisting she had "honored" their agreement and accusing him of "subtly engaging in a tactic that peppers in aggressive comments."
She also claimed Underwood was trying to "monetize" their breakup with a new chapter in his book over which he refused to give her "any sort of approval." A rep for Underwood denied Randolph's claims.
Last month, Underwood opened up about the debacle on the the Reality Steve podcast, asserting that his anger was actually directed at the franchise for taking "advantage" of Randolph. (Reps for ABC and Harrison did not comment.)
"Cass stayed extremely professional and extremely quiet about our breakup," he said. "What bugged me was the fact that that show took advantage of her. I get very defensive over people I love and I know Cass is a big girl and she can fight her own battles, but I just know her heart and she doesn't want to rock the boat and she loves people, even if they abuse her or even if they take advantage of her, which they did."
"I was more frustrated for her, because she just got completely taken advantage of and completely manipulated," he continued. "You have Chris Harrison pointing questions saying, 'I sense you don't want to make Colton mad, or you're afraid you're going to upset Colton. It's like, 'No, Chris. I literally talked to her the morning of that interview. We're good.' Stop worrying about me or painting me to be this controlling or angry person. I'm not angry. If there's anybody I'm upset about or upset with, it's you guys."
Underwood said he had since "distanced" himself from the show.
"I've distanced myself from them, just for my own mental health right now," he said. "There have just been too many things that I've found out about and that has happened for me to feel comfortable working with them on a professional level, not even personal."