Called N-Word by Rockies Fan ...

Miami Marlins outfielder Lewis Brinson was subjected to racist venom during his final at-bat during Sunday's game against the Colorado Rockies ... being called the n-word by a fan.

Brinson was at the plate at the top of the 9th with the Rockies ahead of the Marlins by quite a bit, and with one out logged ... he stepped up to take a hack. After taking a second ball, somebody in the stands started unabashedly hurling the slur as loud as they could.

The man's words were picked up by a mic ... and you can clearly hear him saying it over and over again, seemingly in Brinson's direction -- as the guy turned around to look himself.

It's unclear if the fan was found and punished for the reprehensible…