Music legend is bravely battling a serious brain disorder that’s forced him to cancel his upcoming concerts — and it’s got loved ones fearing the Piano Man may be left with permanent damage, sources say.

Joel, 76, announced that he’s suffering from normal pressure hydrocephalus (NPH), a condition in which excess cerebrospinal fluid accumulates in the brain.

“Billy hasn’t been well for some time. He’s been stumbling around and acting strangely, and he did fall on stage not too long ago,” an insider confides.

— Joel’s 43-year-old wife and mother of his daughters , 9, and , 7 — expressed gratitude for the hitmaker’s “swift” diagnosis, adding, “We are hopeful for his recovery.”

The insider shares, “What he has is not fatal at this point. But the fear, of course, is that the diagnosis has come too late and he’ll have debilitating problems for the rest of his life.”

But sources say the unsinkable singer is tackling his health crisis with sheer will and determination.

“This condition has been exacerbated by recent concert performances, leading to problems with hearing, vision and balance,” Joel’s rep explains. “Under his doctor’s instructions, Billy is undergoing specific physical therapy and has been advised to refrain from performing during this recovery period.”

In February, Joel — who’s also dad to daughter , 39, with ex-wife — toppled over during a show in Connecticut. But he gamely got up and finished the set.

A month later, the “Uptown Girl” singer announced that he was taking a four-month hiatus from performing because of an undisclosed “medical condition” requiring surgery. He’s since canceled all shows through July 2026.

Adds the insider, “People are worried about him — especially with his weight going up and down all the time.”

Dr. Gabe Mirkin, who has not treated Joel, tells the National ENQUIRER that NPH patients may have difficulty walking or maintaining balance, and experience bone-shattering falls.

The physician warns that they may also display signs of brain damage, such as memory loss and behavioral changes — and experience vision or hearing loss from nerve damage.

Mirkin also cautions, “It certainly can cause dementia.”

He further explains, “It is treated often very successfully with a shunt, which opens the pathways for the fluid to pass in greater amounts from the brain into the general circulation. His prognosis really depends on how much damage he may already have — as some of the damage may not be reversible.”