The Moroccan played just nine minutes off the bench, but went on a mazy run from inside his own half- taking out five Baggies defenders - to seal the decisive goal, after Ben Foster had repeatedly frustrated the hosts with a number of saves.
Saints struggle for openings
The hosts almost opened the scoring just before the 20-minute mark when Ryan Bertrand's free-kick clipped the top of the wall and hit the corner of the post, with Baggies' 'keeper Ben Foster beaten.
The Saints full-back then turned provided with his run from deep, as the England defender cut the ball back to Dusan Tadic but the Serbian's shot was tame and straight at Foster.
Salomón Rondón was becoming a spectactor at the opposite end of the pitch for the Baggies with a lack of quality from deep, as Southampton continued to flood the midfield. The biggest incident of note for Tony Pulis' side was the removal of Jonny Evans just after the half-hour mark, after the West Brom skipper sustained an ankle injury.
Minutes before the break Maya Yoshida had efforts on goal from corners, one from a close range header which forced Foster into a instinctive save, but as the whistle blew for the interval - perhaps unsurprisingly - both sides went into half time goal-less.
Second period balances out
Mauricio Pellegrino's men had dominated with almost 70% possession in the opening period, but as has been indicative of the South coast side this term, chances were at a premium once more.
When openings did come, they frequently came from the industrious Bertrand on the left and the second half began in similar fashion.
West Brom had made little impact in the game, but when the Baggies boss brought on Gareth McAuley, the visitors switched to a wing-back system in the hope of injecting life into their game.
It almost paid off within minutes, when Jay Rodriguez was released through the middle, but the former Saints striker's made a hash of his shot and went harmlessly wide.
Suddenly the game had become re-balanced with spaces opening up across the park, with West Brom making the best use of it.
Foster continues to frustrate
Southampton had been thwarted in the opening 45 minutes by Baggies' stopper Foster and the man between the posts for the visitors denied Tadic once more, when the winger was released on the right side of the area. Foster stood up well however, as the Saints attacker's effort was blocked as he tried to curl his shot into the corner of the net.
The addition of substitute Nathan Redmond on the right wing galvanised Southampton in the last quarter of an hour and on this occasion however, Foster was unable to do anything as the England international's dipping, swerving effort went inches past the far post.
West Brom began to inch back toward their own territory as the minutes ticked by and with less than ten minutes to play, a goalless draw seemed to be on the cards.
Moroccan genius lights up South coast
Nine minutes from time, Pellegrino made a double change in bringing on Charlie Austin and Boufal in a final throw of the dice on in the 86th minute, the latter produced a rare moment of genius to send St. Marys' into raptures.
The Moroccan who had been much-maligned since his switch from Ligue 1 last summer, picked up the ball inside his own half and swivelled past Rondon.
The winger than faced down both Grzegorz Krychowiak and Allan Nyom who collided into each other, continuing his mazy run toward goal. Boufal then shrugged off Ahmed Hegazi and McAuley and composed himself, sliding the ball past Foster into the bottom corner.
In truth the game had barely deserved it, but Boufal's may prove to be one of the goals of the season. The goal knocked the stuffing out of West Brom and as the goalscorer picked up the ball again into stoppage time, he almost set up a second for fellow sub Austin, with the striker hitting the outside of the upright after he was dragged wide.
After a largely forgettable 85 minutes, the home fans went home happy with a memory and goal that will long live on for Saints fans.