JoJo Siwa chatted with a 5-year-old cancer patient before surgery … telling her everything would work out and she would kick cancer’s butt.

As you see in the conversation between JoJo and Blakely Bergenfeld … JoJo’s a pro at putting her young fans at ease … patiently answering the little girl’s questions.

Blakely’s parents, Jason and Kimberly Bergenfeld, tell TMZ … the cancer diagnosis was sudden, after the little girl began experiencing headaches and fevers during a recent family ski trip.

We’re told Blakely’s parents took her to a pediatrician when they got back to L.A., and the doctor noticed her liver was swollen and rushed her to the ER at L.A. Children’s Hospital.

Blakely’s dad says his daughter underwent a series of tests, and that’s when doctors discovered cancer in her kidney. She underwent surgery to remove a giant tumor on her kidney Friday.

We are told the surgery went even better than planned. She had to have her kidney removed because it was close to rupturing and would have been terminal if they had not treated it in time. The doctors also found tumors on her liver and lungs, which were removed. The doctors will know in the next few days exactly what stage and type of cancer she has, but she is stable and past the first step on her road to recovery.

It’s been a whirlwind few days … we’re told Blakely had appeared completely healthy before she started feeling ill.

JoJo got involved after learning of Blakely’s diagnosis through ‘Dance Moms’ star Kira Girard and her husband, who are close friends with the Bergenfelds.

Blakely is a huge JoJo fan and was supposed to meet her at the Nickelodeon awards last March before it was canceled due to COVID.