Carolina Panthers wide receiver Tetairoa McMillan is spending his Thursday apologizing to fans after he used the n-word during a livestream ... saying "there's no excuse for what I said."

McMillan issued the remorseful message on Instagram ... saying, "I sincerely apologize for speaking thoughtlessly and will do better."

The first-year player was playing Madden in "The Streamer Bowl," a charity esports tournament that has Twitch streamers and NFL stars on teams as they take on challenges.

T-Mac was streaming with YonnaJay and Tamir when the moment occurred. After intercepting a pass, he shouted the slur in excitement.

The other two streamers appeared shocked after he walked off, with YonnaJay whispering something to Tamir.

The clip spread like wildfire on social media not long after it happened, with some posts garnering millions of views.

Whether McMillan will address it further when he attends NFL Honors tonight remains to be seen, as he is up for the NFL's Offensive Rookie of the Year award.

It was a strong season for University of Arizona product ... racking up 75 catches for 1,095 yards and seven touchdowns in his first year in the league.