JoJo Siwa thinks her girlfriend Kylie Prew has "changed [her] life forever".

JoJo Siwa

JoJo Siwa

The 18-year-old YouTuber posted a sweet message on social media to celebrate five months with her partner, describing her as the "best girlfriend in the world".

She wrote on Instagram: "You’ve changed my life for forever. I love you more and more every day.

"Thank you for being the best girlfriend in the world.”

She added: "Happy 5 months my love bug. (sic)"

Kylie also marked the special occasion as she took to her Instagram Story to share a touching message.

She wrote: "5 months with my absolute favorite person.

"I love you more and more each day (sic)"

In April, JoJo admitted she isn't keen to label her sexuality but she believes that "technically" she is pansexual because "her human is her human".

She said at the time: "I still don't know what I am. It's like, I want to figure it out. And I have this joke. Her name is Kylie. And so I say that I'm Ky-sexual. But like, I don't know, bisexual, pansexual, queer, lesbian, gay, straight.

"I always just say gay because it just kind of covers it or queer because I think the keyword is cool. I like queer. Technically I would say that I am pansexual because that's how I have always been my whole life is just like, my human is my human."

And JoJo feels lucky to have such a supportive partner in Kylie.

She added: "She never cares what the internet said about us. It's nice to have somebody in my life like that ... I told her my whole spiel that I tell everyone when they ask me my life story. She goes, 'I could have Googled that. I want to know your story. You just told me about your career. I want to know about you.' And I was like, 'No one's ever asked me that before.'"