Yet, the new seven-and-a-half-year contract is likely to ward off potential suitors as it includes a €80m (£71m, $98m) release clause which will progressively rise until it reaches €108m.
The 23-year-old forward made his first-team debut in 2014 after garnering a big reputation in the youth ranks of the Basque side.
Williams made an instant impact in La Liga and quickly attracted the attention of bigger clubs due to his impressive pace.
Two years ago, The Guardian first reported that Jurgen Klopp was closely monitoring his development ahead of luring the Under-21 Spain international to Liverpool.
Athletic reacted to that speculation and only days later they tied the Spaniard to a new long-term deal which increased his release clause from €20m to €50m.
However, reports last year claimed that Liverpool, Borussia Dortmund and Juventus were still monitoring his situation amid suggestions that Klopp saw the pacy forward as the new Pierre-Emerick Aubameyang.
In February last year, Williams himself added fuel to the saga after claiming that he wanted to stay at Athletic, but left the door open to a move if a big club came knocking with an interesting offer.
In June, Spanish Publication AS reported that Athletic were eventually ready to offer Williams a new deal less than two years after his previous agreement amid concerns that the likes of Liverpool or Juventus could meet his €50m release clause.
The Basque side have now made sure that Williams will not follow in their footsteps anytime soon by handing him a new long-term contract with a stunning new buy-out clause.
"Athletic Club and Iñaki Williams have signed a renewal agreement by which the player will remain in the rojiblanca discipline until June 30, 2025," the Basque side confirmed through an official statement.
"The cancelation clause will be 80 million euros, progressively increasing to 108 million."
Athletic could yet be about lose another talented player with Kepa Arrizabalaga expecting to join Real Madrid either this month or in the summer.
The Spain international keeper is out of contract at San Mames at the end of the season and Athletic have so far failed to convince him to sign a new deal.
Real were expected to pay the keeper's €20m release clause to sign him during the current January transfer window but latest reports from Spain suggest that Los Blancos will finally wait until the summer to get him as a free-agent.