“Moon River” crooner ANDY WILLIAMS is bravely battling bladder cancer!

The singer, 83, recently left his home in Branson, Mo. – where he still performs – and traveled with wife Debbie to consult specialists at the MD Anderson Cancer Center in Houston, Texas.

“Andy got the diagnosis of inva­sive bladder cancer very recently,” divulged a source close to Williams.

“That means his cancer has invaded the lining of the bladder, but doctors weren’t sure yet whether it had spread to other parts of his body.”

After under­going testing and treatment, he was discharged from room 930 on Oct. 24, and flew back to Branson on Oct. 30 for business meetings. The next day, Williams was slated to return to Houston to begin a three-week course of chemotherapy.

“If the chemo shrinks the tumors, doctors will surgically remove An­dy’s bladder,” noted the source.

“Invasive bladder cancerhas one of the highest recurrence rates of any cancer. So, in a situation such as Andy’s, doctors believe it’s best to remove the whole bladder.”

After a second round of chemo in Houston, Williams – perhaps best known for his weekly TV variety show in the ’60s – and his wife will move into a rental home in Malibu, said the source.

“Andy is in for the fight of his life, and he wants to be closer to the hospitals in L.A. that have top-flight cancer treatment centers,” the source divulged.

“He’d like to come back to Branson for his annual Christmas show at his Moon River Theater.”

But that now seems unlikely. In fact, his website has already announced: “Due to health reasons Andy may not make a live appearance in his Christmas Show.”

Added the source: “His wife Debbie told him, ‘You have to beat this thing first!’"