The striker, who was recently involved in a dressing room bust-up, was videoed taking the potentially harmful 'laughing gas', with the club condemning his actions
The video was reportedly filmed after West Brom's 3-0 defeat at the hands of Manchester United on March 8.
Although not illegal, nitrous oxide, often referred to as 'laughing gas', is potentially harmful to the body due to a lack of oxygen that can occur after its use and sporting and technical director Richard Garlick condemned Berahino's actions.
"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 released by the club.
"Saido knows he has made a mistake. He has apologised for his actions and has assured the club it will not happen again."
The England Under-21 international insisted the incident was not something he would be repeating.
"This was very poor judgement on my part and not the right example to be setting," he said.
"At the time, I wasn't fully aware of the serious health risks involved but now I know, I won't be doing it again."