Bruce Arians will have to pay up for slapping his own player's helmet during last weekend's playoff game ... 'cause the NFL is fining him $50k for the incident.
The moment happened during the Bucs vs. Eagles Wild Card game on Sunday ... after Philly receiver Jalen Reagor fumbled a punt return and Tampa Bay recovered.
During the pile up, Bucs safety Andrew Adams appeared to try to get Philly players out of the way ... which pissed Arians off to the point of striking the 29-year-old in the helmet.
The whole thing was caught on camera during the broadcast .