Jeremy Piven... who?

Jeremy Piven's so-called star power FAILED to get him out of a ticket yesterday -- because a cop wrote him up in Los Angeles ... a cop who claimed to have no idea who the hell Jeremy Piven is.

The cop wouldn't tell us why he pulled over the "Entourage" star -- but Jeremy insists it was just a registration issue and told us, "there's no problem actually."

After the cop wrote up the actor -- and handed him a citation -- we asked the cop if Piv was the biggest star he'd ever pulled over ...but the officer claimed he had no idea who Jeremy was.

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