The centre-back smashed the ball in after a deep free-kick was weakly cleared, giving him the chance to volley home.
The goal was Elustondo's sixth for Sociedad in 65 games.
Real Sociedad have lost to Barcelona just once in the last eight years when the match has taken place at the Estadio de Anoeta.
Marc-Andre Ter Stegen was rooted to the spot as the strike sailed past him.
The goal was the first one scored at the newly-renovated stadium, which has had the stands moved in closer to the pitch to help generate atmosphere.
15 September 2018