The 23-year-old appeared to make an offensive gesture towards the fourth official as he left the pitch during Arsenal's Champions League clash with Bayern Munich
The Pole was sent off by match referee Nicola Rizzoli during the first half of the Champions League last-16 first leg tie at the Emirates Stadium after bringing down Arjen Robben, who was through on goal.
David Alaba subsequently missed the spot-kick but the Arsenal goalkeeper caused further controversy as he left the pitch.
Szczesny made the gesture, which was caught on camera, as he headed for the dressing room down the tunnel at the Emirates.
Sky Sports, who broadcast the game, apologised to their viewers for showing the 'untoward gesture.'
The Poland goalkeeper will now face a suspension for his initial sending off, though Uefa could well take additional action for his response to the incident.