Black Surfer Claims Board Stolen, Nailed To Tree In Racist Act, Cops Investigating

A Black surfer in Florida says several of his white counterparts stole his board and nailed it to a tree in a racist act ... and the allegations are so disturbing, cops tell TMZ Sports they're now investigating the matter.

The surfer's name is Andrew Sherlock Mills ... and on Tuesday, he explained in a social media post that the incident went down last weekend in Jupiter.

He claimed that when he turned his back after riding in the ocean waters, his board was nabbed and then pinned high up on a nearby palm tree.

"This is how real the racism and hate is in Jupiter, Florida," Mills wrote in an Instagram post. "They can't stand to see a Black man in their waters. I was told I'm not welcome. Stole my board when I turned my back and they did this. Said it was a message. They nailed it to the tree like a lynching."

Mills said he did not call cops ... and instead, "took the board down, stayed and surfed the whole day with a small board to show face."

"A few locals that were there were apologizing and agreed it was wrong," Mills added. "They did said, after what happened, I'm welcome to surf their waters freely and no one will bother me. I'm just going to leave it in God's hands."

We reached out to the Jupiter Police Dept. for comment on the situation ... and a spokesperson told us officers were made aware of the claims through social media and have since launched a probe.