Fermín López Marín was part of the Campillo youth setup in his home town, and then signed for Recreativo Huelva. After one season, in 2011/12 he moved to Real Betis Balompié and four years later entered La Masía as an U14.

He rose through the age ranks and eventually played for the Spanish championship winning U19A team.

In 2022, Barça loaned him out to Linares Deportivo in Primera Federación so that he could gain playing time and experience. Fermín did just that and was top scorer for the team with 12 goals and 4 assists.

It was enough to earn a return to Barça for the 2023/24 season and for Xavi Hernández to choose him to go on the US pre-season tour with the first team, where he performed admirably in the Clásico against Real Madrid in Dallas, scoring a truly special goal and providing an assist in the 3-0 win.

After being named in several of Xavi's squads and starting XIs, he was registered as a first team player in 2024. 

His excellent campaign led to a call up from the full Spanish national side and he made his debut in a friendly against Andorra. The midfielder made the squad for the victorious Euro 2024 tournament, making an appearance in the group stage against Albania. Fermín's dream summer continued as he was top scorer with six goals in the men's football tournament at the 2024 Paris Olympic Games, helping Spain to the gold medal.