Odell Beckham Jr. -- Hey LeBron ... Watch Me Dunk!!

In yet another display of freakish athleticism, here's New York Giants superstar wide receiver Odell Beckham Jr. dunking like a boss ... while wearing a backpack.

The NFL star -- who was just named offensive Rookie of the Year -- put on an impromptu slam dunk clinic on an outdoor court earlier Tuesday ... and gave a shoutout to LeBron James while he was at it.

Fun Fact -- Beckham Jr. says soccer is his 2nd sport ... not hoops.