The 'Dance Moms' star shared her praise for Siwa on Instagram.
"I always knew the world would be a more colorful, positive, sparkly place with a kind, loving, dazzling triple threat like you in it - a shining example for the kids out there to live their BEST lives each and every day," Miller wrote alongside a slideshow of pics of herself and Siwa. "*You put your heart on the platter! (If you know you know)."
"Love you kiddo!!! Keep making me proud 🌈," she added. "#aldcalways #ALDC #abbyleedancecompany #losangeles #aldcla #LA #pittsburgh #aldcpgh #PA #jojo #jojosiwa #loveyou."
GLAAD President & CEO Sarah Kate Ellis also praised Siwa in a statement on Saturday.
"JoJo Siwa's decision to share her truth with the world is a powerful moment that is being celebrated by the millions around the world who admire her," she said. "As one of the most influential young role models today, JoJo's story is a reminder for LGBTQ youth to love who they are and to find safe and welcoming environments to speak out."
In an Instagram video on Saturday, Siwa opened up about her sexuality, and thanked her followers for "the most endless amount of love and support."
"Somebody said, 'What label are you?' And you know, I have thought about this and the reason I am not ready to say this answer is because I really don't know this answer," Siwa shared. "I think humans are awesome. I think humans are really incredible people."
"I want to share everything with the world, I really do. But I also want to keep things in my life private until they're ready to be public," she added. "What matters is that you guys know that no matter who you love, that it's OK and that it's awesome, and that the world is there for you."
Siwa appeared on seasons 5 and 6 of Dance Moms with her mom, Jessalynn Siwa, before signing a contract with Nickelodeon.
The performer has stayed close with Miller in the years since, and often defended her in the public eye. In an interview with ET last June, Siwa explained her close connection to Miller.
"Listen, Abby is Abby ... you either go to Abby's studio, you work for Abby; Abby doesn't just go pick up people off the street and be like, 'You're working for me.' That's not how it works," Siwa said. "You go to Abby, you know what you're getting into. So it makes me really, really upset that I am one of the few -- there's only a few people -- that really truly thank Abby, and are thankful and grateful for what she's done for us. Because without her, Dance Moms wouldn't be a thing."
"And without that, none of us would have a career. Like, Dance Moms is where I got my start," she added. "That's where Maddie [Ziegler] got her start, that's where Kenzie [Ziegler] got her start, Kalani [Hilliker], Paige [Hyland], Brooke, Nia [Sioux], Chloe [Lukasiak], Kendall [Vertes]. I mean, that's where everyone got their start."
See more in the video below.