Jalen Brunson's game-winning shot to send the Knicks to the second round of the NBA playoffs sent New York into a frenzy ... including former star Stephon Marbury -- who went bonkers over the massive victory!!
The Brooklyn native went live on his Instagram for the final few minutes of the Knicks' Game 6 matchup against the Detroit Pistons ... and we're happy he did -- 'cause the reaction was priceless.
Like the rest of New York, Marbury was on Cloud 9 when Brunson hit the go-ahead three-point shot with 4.