Irving was walking on the outside of the Toyota Center before the Houston Rockets beat the Celtics 127-113 when the sequence unfolded.
The five-time All-Star was approached by a man as he was about to board the team bus. He reached down in his pants and pulled out the money, as a security guard kept watch.
Irving then handed the cash to the man, who screamed and threw his arms up in celebration. The man also followed Irving to the bus and gave him a thank you.
Thomas Moreno caught the act of charity on video. He said Irving gave the man a couple hundred bucks.
Irving went on to score 23 points in the loss. He also had 11 rebounds and five assists against the Rockets. The Celtics guard is averaging 23.3 points and career-highs in assists (6.4) and rebounds (5.1) per game this season.
Irving is making more than $20 million this season and has a $21.3 million player option in 2019.