The 43 year-old has overseen a stunning resurgence from the LaLiga side since taking over from Eduardo Berizzo on 4 December.
Berizzo was relieved of his duties after the club followed up a victory in its opening match of the season with a winless run spanning 13 league games.
However, Garitano has since overseen nine wins, five draws and only two defeats in LaLiga.
That run has propelled Athletic into the race for a Champions League qualification place. The cub sits four points behind fourth-place Getafe ahead of their clash on Monday (AEDT).