The pair was sitting courtside at a Los Angeles Lakers game when a pal alerted Piven to the fact his famous neighbour hadn't secured his facial hair with enough glue.
The Entourage star was hesitant to point out the renegade moustache to the Sweet Caroline singer - but insists Diamond was a good sport.
Piven explains, "I look to my right and I realise, indeed, Neil Diamond had a fake moustache on and... it was almost all the way off his face.
"(I said), 'Excuse me Mr. Diamond, your moustache is falling off.' I don't know any other way to say it. So he ended up ripping the rest of it off.".