Following loan spells with Mallorca, Villarreal and Getafe, the 21-year-old left Real Madrid permanently to join Real Sociedad, where he has established himself as a star player over the last 18 months.
Kubo was a standout performer for the Txuri-Urdin last season, scoring nine goals and registering seven assists in 35 La Liga appearances to help the club finish fourth and qualify for the Champions League.
The Japan international has since chipped in with six goals and four assists in 27 games across all competitions so far this term.
Kubo's original contract at the Reale Arena was due to expire in June 2027 and allegedly included a €60m (£52m) release clause.
The winger's impressive form with Sociedad is understood to have caught the eye of former club Real Madrid and Manchester United, who have both been tipped to make a move for him this summer.
Despite the reported interest from two of Europe's biggest clubs, Kubo has decided to commit his future to Sociedad by penning a new long-term deal until 2029.
"I am very happy to be able to renew with this great club, which has welcomed me very well and in which I am very happy," Kubo told Real Sociedad TV.
"My wish is to continue growing hand in hand with this team... For many things, but above all for the good feeling I have with my teammates, with the fans, with the sports management, with everyone.
"I want my renewal not to be the news of the week, I want it to be the victory in the Parc des Princes (against Paris Saint-Germain in the Champions League) and we will give everything to make it that way."
"I was born very far away, but the Japanese and the Basques have a lot in common, so thank you for making me feel at home," Kubo added.
Kubo is in contention to start for Sociedad when they travel to France to take on PSG in the first leg of their Champions League last-16 tie on Wednesday - the club's first knockout match in the competition for 20 years.