David Cassidy Says It Wasn't Alcohol, It Was Dementia

David Cassidy says it may have looked like he fell off the wagon at a concert Saturday night, but it wasn't that ... he says he has dementia.

Cassidy seemed intoxicated at the concert outside L.A., falling off the stage, forgetting words and unsteady on his feet.

The singer now says he was diagnosed with early-stage dementia ... first reported by People. Sources close to David tell us he has said he was not drinking before the concert.

Cassidy's mom and grandfather struggled with the disease and he says he went to a clinic and was recently diagnosed.