The incident took place on March 8 following the Baggies' 3-0 defeat to Manchester United, with a video showing Berahino puffing the laughing gas from a balloon before slumping over the steering wheel.
Despite nitrous oxide not being illegal, West Brom sporting and technical director Richard Garlick released a stern statement warning the 20-year-old about his conduct.
"I have spoken to Saido about the incident and told him in no uncertain terms that his conduct is not befitting a West Bromwich Albion player," Garlick said in a statement.
"Saido knows he has made a mistake. He has apologised for his actions and has assured the club it will not happen again."
Berahino was involved in another controversial incident at the end of March when he was allegedly involved in a dressing room fracas with teammate James Morrison following his side's 3-3 draw with Cardiff City.