Diego Colotto bagged the game-winner in the 77th minute.
Erreala now sit in 15th in the table with a record of three wins, six losses and one draw. Espanyol are in 16th place with a record of two wins, five losses and three draws.
Espanyol gained a win even though Real Sociedad managed to take more shots on goal during the course of play. Real Sociedad fired in 16 shots — eight on target — compared to Los Blanquiazules's shots on goal total of 2.
Erreala controlled possession for about half the contest thanks to strong open field play. Real Sociedad won 69 duels and 18 headers, which led to five shots, including eight on target. Diego Ifrán made a big difference on the match after coming on for Xabi Prieto in the 80th minute, as he took two shots. Carlos Vela had five shots for Real Sociedad, more than any of his team-mates.
Real Sociedad and Espanyol ended the first in a near even split in terms of ball possession, a stat that was reflected in the 0-0 draw at the break. Real Sociedad recorded eight shots with six on net over the match's first 45 minutes, while Periquitos managed four, with one on target.