Imanol Alguacil's Real Sociedad, in excellent form, moved six points behind leaders Barcelona, who face Real Madrid in the Spanish Super Cup on Sunday.
La Real are only three points behind Los Blancos, albeit having played an extra game than the top two, currently in Saudi Arabia.
Alexander Sorloth, Takefusa Kubo and Mikel Oyarzabal's goals helped the hosts beat their rivals at the Reale Arena.
Oihan Sancet struck for Athletic, who had Yeray Alvarez sent off in the second half for a foul on the superb Kubo.
Ernesto Valverde's team have had a good start to the season themselves and defeat leaves them eighth, just two points behind the top four.
However it was a frustrating night for Athletic against their neighbours, whose movement in attack was too tough to handle.
Sorloth netted his fourth goal in four consecutive league games to break the deadlock after 25 minutes.
The Norwegian stayed onside to control Igor Zubeldia's header, sent back into the danger area, and then finished with aplomb.
Their second goal was brilliantly worked, with veteran playmaker David Silva winning the ball back and feeding Kubo.
The Japan international nutmegged Dani Vivian and then fired confidently beyond Unai Simon to double his team's lead.
Sancet pulled one back three minutes later with a smart finish beyond Alex Remiro, to keep Athletic in the game.
However, Alvarez was dismissed for felling Kubo and Oyarzabal netted calmly from the spot, his first goal since returning from injury at the end of December.
The final half hour saw La Real control the ball and Athletic forced back, simply trying to stop their swaggering neighbours from extending their lead.