Justin Bieber's Hamster -- DIES

Justin Bieber's hamster PAC -- the one he gave away to a random screaming fan less than three months ago -- is now spinning in that great hamster wheel in the sky ... in other words, he's dead.

TMZ broke the story ... Bieber unloaded the pet on an unsuspecting teenage girl in Atlanta last December ... cage and everything ... following his Jingle Ball concert. He told her, "That's all you ... you gotta take care of PAC."

The girl -- 18-year-old Tori McClure -- promised to take good care of the rodent ... and she actually did ... until the poor bastard kicked the bucket this week.

Tori tells TMZ, the hamster started to lose its hair two weeks ago so she took it to the vet over the weekend, but it was too little, too late.

We did some rodent research ... hamster hair loss can be caused by a variety of ailments, including chronic renal failure or a low protein diet ... it's unclear exactly what was eating PAC.

Tori says she buried PAC by a tree in her backyard, telling TMZ, "It's very peaceful there."

Unlike a Bieber concert.