JoJo Siwa is a 20-year-old star known for her work with Nickelodeon. ... JoJo Siwa at "The LEGO Batman Movie" premiere in California, on February 4, 2017. David Livingston/Getty Images ...

Page Six has exclusive sneak peek look at Monday’s “So You Think You Can Dance,” where host Cat Deeley announces JoJo Siwa as the surprise third judge with Maks Chmerkovskiy and Allison Holker.