Striker Berahino, who was passed fit to start the match at The Hawthorns despite carrying a knock, gave Southampton goalkeeper Fraser Forster no chance with an unstoppable half-volley after just two minutes.
However, West Brom boss Tony Pulis will be concerned by the fact his top scorer had to be replaced shortly after the break, with Berahino's strike partner Brown Ideye also apparently picking up an injury late on.
Before the game, Pulis stated that his side were still involved in a fight to avoid the drop despite their improved form.
But they looked far from relegation fodder in a solid first half showing that was illuminated by Berahino's 12th league goal of the season.
Southampton arguably edged the second half and out-of-form striker Graziano Pelle wasted a golden chance to equalise with 22 minutes remaining.
Victory sees West Brom climb above Everton into 13th, while Southampton - who are now winless in three Premier League games - stay fifth, but will drop a place if Liverpool beat Manchester City on Sunday.
West Brom, unchanged from last weekend's 0-0 draw against Sunderland, opened the scoring in style early on.
Chris Brunt's long free-kick was headed down by Craig Dawson and Berahino finished emphatically into the top right-hand corner.
Southampton responded to the early setback with a good spell of possession, but West Brom's front two of Berahino and Ideye were proving a menace for the visitors.
Another West Brom set-piece almost paid dividends when Dawson climbed well to head Craig Gardner's header at Forster.
Ronald Koeman, who made five changes to last week's 2-0 defeat to Liverpool, opted to make a tactical substitution in the 25th minute with Eljero Elia replacing Florin Gardos.
Southampton looked more of a threat going forward as a result, but they were almost caught out by Brunt's wonderful long ball that Gardner failed to keep under control in the penalty area.
There was a change of referee at half-time as Phil Dowd, who looked in discomfort as the break approached, was unable to continue and Graham Scott took over in the middle, while Southampton introduced Steven Davis for Victor Wanyama.
Ideye could have done more to punish some lax Southampton defending, before Berahino's afternoon came to a premature end with Youssouf Mulumbu coming on in his place.
As the half progressed, Southampton turned the screw in search of an equaliser and Pelle appeared to have an easy tap-in inside the six-yard box in the 68th minute, but the Italian somehow failed to connect with the ball.
As the clock wound down, Pelle hit a weak volley straight at Ben Foster, before Ideye had to be helped from the pitch to provide Pulis with another striking headache.
