Julianne will serve as a guest judge for Dancing with the Stars' upcoming finale on Monday night at 8PM ET/PT on ABC after Derek tested positive for a breakthrough case of coronavirus, Entertainment Tonightreported.
Derek, however, will reportedly still participate in the big finale through a pre-taped segment and by appearing virtually.
Julianne hinted at her temporary gig Monday via Instagram Stories, when she told her followers, "I'm super excited because today you may see me do something that you've seen me do in the past -- but it's a surprise."
Julianne previously won the mirrorball trophy on back-to-back seasons of Dancing with the Stars, the show's fourth and fifth editions. She later returned to judge the series for several seasons before leaving in 2017.
Many Dancing with the Stars pros and celebrities told ET that they're thrilled to have Julianne be a part of the Season 30 finale.
Pro dancer Jenna Johnson said she is "so excited" about Julianne's return, adding, "Julianne is the reason I started doing ballroom! So to have her judge this finale just means the world."
And pro Cheryl Burke gushed, "I've known Julianne since she was 10 years old, so we go way back."
"It's always nice to have old contestants or old pros come back on our show and judge, so she is amazing and she knows what to look for," Cheryl said. "She's a great replacement for Derek, so I'm really excited."
Derek took to his Instagram last week and announced, "Hey everyone, I have news to share and I wanted you to hear it straight from me. Even though I've been fully vaccinated, I've just been diagnosed with a breakthrough case of COVID. I just found out."
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"And I feel okay. I feel strong, but I'm currently taking advice from medical professionals, doing everything I can to get better as fast as I can," he noted.
Derek postponed his Las Vegas shows last week and planned to rest and stay at home.
He added in his Instagram post, "I'll keep you all updated with what's going on. But I just wanted to send a lot of love out there to you all and, yeah, stay safe."
Derek already missed one Dancing with the Stars broadcast in early October due to a potential COVID-19 exposure.
"Due to a potential COVID exposure, and although he is fully vaccinated and tested negative, Derek will sit out tonight's show out of an abundance of caution," Dancing with the Stars host Tyra Banks announced at the top of the October 4 episode.
And earlier this season, both Cheryl and her celebrity partner, Peloton instructor Cody Rigsby, tested positive for breakthrough coronavirus cases one after the next.
When Cheryl was battling COVID-19 in late September, she and Cody were judged based on a previous in-studio practice performance and ultimately survived elimination.
Cody tested positive for COVID-19 days after Cheryl, and so the couple also danced their routine for "Britney Night" -- which paid tribute to pop icon Britney Spears -- live via videochat from their respective homes.
Not only does Derek serve as a judge onDancing with the Stars, but he also previously judged World of Dance, which was executive produced by Jennifer Lopez, and is six-time mirrorball trophy champion on DWTS.