On 24 September 2020, Martínez (at that time heavily linked in the media with a return to Athletic Bilbao) scored in extra-time to win the 2020 UEFA Super Cup for Bayern Munich with a 2–1 victory over Sevilla; it was his second goal in two appearances in the UEFA Super Cup. Having scored the decisive goal once again in the extra time for Bayern as he did earlier in 2013 against Chelsea in the UEFA Super Cup final, he famously earned the nickname of "Mr. Super Cup" (later made famous in the sports media) from his teammate Thomas Müller who became man of the match. On 4 May 2021, Bayern announced that Martínez would be leaving Bayern at the end of the season, since both parties agreed not to extend his contract.