Kyrie Irving and Celtics fans are still beefin' ... the Nets star point guard gave double middle fingers behind his head to a group of fans who were heckling him during Sunday's game.
Celtics fans were all over Kyrie -- who spent two seasons with Boston before signing with the Nets -- trying to get him off his game ... and Kyrie seemingly had enough and responded.
In the short clip, you see Irving flip not one but two birdies behind his head as he stands along the baseline, waiting to receive the ball from the ref before inbounding the ball.
Of course, Irving's double bird drew a response from the crowd ... who was happy they finally got KI to respond.
After the game, Irving addressed the incident with the C's fans ... saying, "Where I'm from I'm used to all these antics and people being close nearby. It's nothing new when I come into this building what it's gone be like. But it's the same energy they have for me, I'm gonna have the same energy for them."
He continued ... "It's not every fan. I don't want to attack every Boston fan. When people start yelling p**** or bitch and f*** you and all this stuff, there's but so much you can take as competitor. We're the ones expected to be docile and be humble and take a humble approach. Nah, f*** that, it's the playoffs."
It's not the first time Kyrie's gotten into it with the Boston fans. You may recall, he compared C's fans to a "scorned girlfriend" after they chanted "Kyrie sucks" late in the fourth quarter of a game back in March.