The hosts had failed to score in their previous three games, but they looked threatening from the outset and created two clear chances in the opening four minutes.
First, Aymeric Laporte threw himself at Alex Berenguer’s cross and flashed a header wide of the far post, before Berenguer’s low effort from 15 yards was saved by Pau Lopez, who then smothered Iñaki Williams’ follow-up.
That long-awaited goal finally arrived in the 25th minute, and Athletic had an unlikely source to thank, as Williams set up centre back Dani Vivian to open his account for the season with a low effort into the far corner from the edge of the box.
Betis had thrashed Panathinaikos 4-0 in the UEFA Europa League less than 72 hours earlier, but they were toothless here, as manager Manuel Pellegrini cut a frustrated figure on the touchline in his 300th game in charge.
And the Chilean’s mood would have darkened further on the stroke of half-time when Williams claimed another assist, with the striker springing the offside trap and pulling the ball back for Oihan Sancet to tuck his fifth goal of the campaign into an empty net.
In response to Athletic’s lead – only their second in 15 LaLiga games – Pellegrini made a triple change at half-time, bringing on Hector Bellerín, Sergi Altimira and Pablo Fornals for Angel Ortiz, Marc Roca and Antony.
But Los Verdiblancos nearly went 3-0 down nine minutes after the restart, when López flew to his left to tip Berenguer’s long-range strike over the crossbar.
Yet, while Athletic continued to look dangerous, Betis also had their moments, and Abde Ezzalzouli forced a smart save from Unai Simon when he raced into the box and fired low towards the near post.
That was a warning Athletic failed to heed, and Fornals provided a potential route back for Betis 15 minutes from time, as he curled a stunning free-kick into the top corner from 20 yards, taking his goal tally to seven for the season.
Cedric Bakambu then prodded beyond Simon two minutes later, but the substitute was denied a fourth goal involvement in six H2Hs after VAR ruled he had strayed offside.
Betis’ late rally made it a nervy finale for Athletic, but the hosts saw out a victory that sees them climb to ninth in the table – but just three points shy of the European places. As an added bonus, they can also celebrate a first league double since 2017/18 over Betis, who remain fifth but are now winless in five games in LaLiga (D3, L2).
Flashscore Man of the Match: Inaki Williams (Athletic Club)