Midfielder Fran Yeste doubled Bilbao's lead in the 75th minute with a curling left-footed shot from a free kick on the edge of the penalty area.
Substitute Roberto "Tiko" Martinez sealed the victory in the third minute of injury time by stepping forward and firing a low shot from just inside the area.
The game at Bilbao's San Mames stadium marked the 100th anniversary of the club's first meeting with Sociedad. The teams' first Spanish league match came in the 1928-1929 season when the competition began its existence.
Bilbao, which stays in 10th place, has 44 points and retains a chance of a European place next season. Sociedad is 12th with 39 points.
Earlier Saturday, striker Miguel Lopez scored in the 86th minute to earn last-place Numancia a 1-0 victory over Atletico Madrid.
Barcelona has 69 points, followed by Madrid with 60. Villarreal is next with 50 and a game in hand. Betis, Espanyol and Sevilla have 49 points each, two more than Valencia.
Kevin Kuranyi snapped his scoring drought with a hat trick that stunned Schalke 3-0 and kept Stuttgart's title hopes alive.
The Germany forward ended a well-publicized six-match scoreless streak and charged around the pitch in relief after his first goal.
"I'm so happy -- I thank everyone that believed in me, especially God," the Brazilian-born German said.
The loss knocked Schalke from its share of first in the table with the title race down to six matches. It dropped the club three points behind Bayern Munich, while Stuttgart remained five off the lead.
An 84th-minute header from Michael Ballack allowed Bayern to charge back from a goal down to beat Borussia Moenchengladbach 2-1. The front-runners also got a boost from reserve Mehmet Scholl, who leveled in the 66th minute.
Kuranyi sent 48,600 fans at Stuttgart's Gottlieb-Daimlerstadion into a frenzy after he tapped in from close range in the 15th minute to end 569 minutes of futility.
Kuranyi followed with two more goals in the 47th and 63rd minutes, while teammate Alexander Hleb shone in the midfield, as Stuttgart dominated the match.
"We made some mistakes," Stuttgart coach Matthias Sammer said. "But we had two players up front that were world class today, Kuranyi and Hleb."
Stuttgart won for the fifth straight time.
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