The 28-year-old has been barred because he has a conviction for assault.
But a photograph posted on Twitter by a team mate shows Barton, who said he "regretted" the ban, on holiday in Los Angeles in June.
Newcastle United declined to comment on Barton's trip, saying the holiday was not a "club issue".
The US Embassy said it did not comment on individual cases but confirmed anyone with a criminal conviction had to declare it and needed a visa to travel.