The 26-year-old, who has struggled with a thigh injury this season, has put pen to paper on a new three-year deal at Anoeta.
Agirretxe came through Sociedad's youth system and made his first-team debut in 2005. He enjoyed his most prolific campaign to date last season, notching 13 goals in La Liga.
However, partly due to his recovery from injury, Agirretxe has scored just once this season - a late equaliser in a 2-2 draw with Cordoba in the Copa del Rey.
"I have fulfilled my dream. I hope to have more special years here," he told the club's official website.
"The club wanted me to stay here and I feel very important and valued."
‘Even though Kevin Keegan was my boyhood hero, we never really saw eye-to-eye. That was an underwhelming time for me – I felt on the outside’: Newcastle cult hero reveals disappointment over how things went under King Kev
Alexander Isak puts uncertainty over his Newcastle United future to bed amid fresh PSG and Arsenal links
