According to reports in Spain, now confirmed by Sky Sport in Germany, the 32-year-old has a general agreement to rejoin Athletic.
They point out that no official offer has been made to Bayern yet, but the German club will not stand in his way.
Martinez joined Athletic from Osasuna as a 17-year-old with the club investing €6m in him.
Following six great seasons at the Basque club, Bayern Munich triggered his €40m release clause in 2012.
Despite regular injury problems, the Spain midfielder has been a huge success in his eight seasons at Bayern.
Martinez played an important part in eight Bundesliga title wins and two Champions League triumphs for the Bavarians.
Now 32, Martinez is no longer a key player at Bayern and the club will not stand in his way.