NHL star Tyler Seguin trash-talked the police in Cape Cod, MA ... right behind their backs ... saying, "F**k the 5-0" -- moments after shaking their hands ... and TMZ Sports has the footage.
The 21-year-old Dallas Stars player was hangin' out at a 4th of July party in Cape Cod back in the summer ... when cops responded to a noise complaint.
When the footage begins, it seems Tyler has smoothed things out with the cops ... who are smiling and laughing with the NHL player ... before exchanging goodbye handshakes.
But as the cops turn the corner to leave, Tyler turns to his friends and whispers, "F**k the 5-0."
He then asks the group if they should start a "f*ck the po-po" chant ... to which everyone cries "no!"
Interesting fact -- the incident took place on the same weekend Seguin was traded from the Bruins to the Stars ... after Boston got fed up with his bad boy antics.
Seguin has performed a sort of image makeover since joining the stars ... the incident with Cape Cop police seems to be the last rebellious thing he did before cleaning up his act.
We reached out to Tyler for comment. So far, no word back.