The 18-year-old YouTube Star shared snaps of the two cuddling and wearing matching Gucci jackets.
"You've changed my life for forever," she wrote. "I love you more and more every day. Thank you for being the best girlfriend in the world. Happy five months my love bug."
On her Instagram Story, Prew also shared a photo of her kissing Siwa on the forehead for the occasion, writing, "Five months with my absolute favorite person. I love you more and more each day."
Siwa came out as a member of the LGBTQ+ community in January. In a chat on Tuesday with Instagram head Adam Mosseri, she said it was actually harder for her to tell her mom, Jessalynn, that she no longer wanted to wear her hair in her signature bow style than to come out to her. She also explained the style decision, which she shared with fans in a TikTok video in May set to Shania Twain's "Man! I Feel Like a Woman!"
"I just had my 18th birthday and it was right around my birthday that I realized, 'I kinda wanna do my hair different today,'" she shared. "It just felt like the right time and to do something different and to do something maybe a little bit more mature or a little upgrade."
"The bow is still a part of my life," she added. "I still love bows, I forever will. They're who I am, but maybe I won't wear it every day."
Last April, Siwa talked to ET about her fans going into a frenzy when she first took out her signature bow and high ponytail to reveal her natural waves.
"It's hilarious," Siwa said of the massive response from fans. "I knew it would be a shock to people ... I knew it was going to be like, 'Oh my gosh, JoJo put her hair down,' because people just don't see that. But I also did not expect it to go as [viral] as it did."
"The bow will never be over," she continued. "I mean, the bow is my life. I loved it ever since I was little. It's become who I am."