EXCLUSIVE

Von Miller's fishing expedition in Florida has ignited an investigation after PETA accused the NFL star of illegally catching a hammerhead shark ... TMZ Sports has learned.

Miller and some friends went fishing in Miami last week and reeled in the 9-and-a-half foot shark -- which they later threw back in the ocean.

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PETA -- People for the Ethical Treatment of Animals -- went berserk over the footage ... accusing Von's group of illegally catching and killing the animal.

PETA vowed to follow up with wildlife officials and said they would "ensure Miller is held accountable."

The complaint got all the way to the Florida Fish and Wildlife Conservation Commission -- a government law enforcement agency that regulates hunting and fishing.

We spoke with FWC Public Information Coordinator Rob Klepper who tells us, "FWC has received the images and video and is looking into whether or not a violation occurred in this incident."

Unclear what the punishment would be if officials found Miller broke any laws -- we're looking into it.