Cristian Portu scored the only goal as Real Sociedad edged Athletic Bilbao 1-0 in a disappointing Basque derby on Thursday.
Portu found the net after just five minutes at San Mames, finishing off a counter-attack by Mikel Oyarzabal.
Sociedad remain in third place in La Liga with 29 points, four behind Real Madrid who have a game in hand.
Atletico Madrid still lead the way at the end of 2020 on 35 points, two ahead of their city rivals.
Victory for Sociedad was also a boost with the two sides possibly facing each other in the Spanish Super Cup final in January.
They will also meet in the delayed 2019/2020 Copa del Rey final in April.
"We were very clear about how we had to play and how we could harm them, they did not have any clear chances and we were able to kill the game," said Portu.
"By results I would say that it has been a very good year, but we have a bittersweet taste for all the fans who have not been able to enjoy our victories in 2020."