Golf Channel and The Guardian reported details of the entanglement, which came not long after the European Ryder Cup team buried the United States squad in a 17 1/2 - 10 1/2 victory on Sunday.
The American golfers nearly came to blows, The Guardian reported, and were separated by others nearby.
The source of the friction was not known.
Johnson and Koepka are good friends who work out together at trainer Joey Diovisalvi's Joey D Golf Performance Center in Jupiter, Fla.
Johnson and Koepka partnered in the Saturday afternoon foursomes, losing to Justin Rose and Henrik Stenson.
--Field Level Media