Gerard Martin, Marc Casado, Ronald Araujo and Robert Lewandowski scored a goal each in the easy home win that also takes them three points clear of third-placed Real Madrid, who lost 2-1 at Real Betis on Saturday while Atletico were beating Athletic Bilbao 1-0 at home.
"Very happy with the match we played," Araujo said after Sunday's victory.
"It was important to win and become leaders again. It was a great effort by the team."
Sociedad played with a man down from the 17th minute after Aritz Elustondo was sent off with a straight red card for grabbing Barcelona forward Dani Olmo to stop a breakaway.
"It was a clear red card," Sociedad coach Imanol Alguacil said.
"After that, there was no game. It's already hard 11 against 11 against them, so you can imagine with a man down."
It was the sixth win in a row for Barcelona in La Liga, and the second loss in three matches for Sociedad, who stayed in ninth place.
Barcelona coach Hansi Flick rested some regular starters ahead of the team's Champions League match at Benfica on Wednesday in the first leg of the round of 16, but it made no difference to their efficiency.
Barca were in control from the start at their Montjuic stadium, with Martin scoring in the 25th and Casado four minutes later.
Araujo added to the lead in the 56th while Lewandowski closed the scoring in the 60th for his eighth goal in his last nine matches.
Lewandowski now has 34 this season in all competitions, one more than the 33 he scored in what had been his previous best season with the Catalan club in 2022-23.
Sociedad, who along with Barcelona, Madrid and Atletico are playing in the Copa del Rey semi-finals, struggled from the start and had no attempts on goal, while Barcelona had 33.
"There is still a long way to go and everything can change very quickly," Flick said.
"We have to keep fighting until the end because we want to win everything we can."
Elsewhere, striker Diego Garcia scored two minutes into second-half added time to give 16th-placed Leganes a 1-0 win over 14th-placed Getafe.
Eighth-placed Mallorca drew 1-1 with 19th-placed Alaves and Valencia missed a chance to move out of the relegation zone after a 3-3 draw at mid-table Osasuna.
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