Alfa Semedo was born on 30 August 1997 in Bissau, Guinea-Bissau. He spent time in Benfica's under-19 setup before senior spells at AVS and Moreirense, where his performances earned a return to Benfica.

He signed a five-year contract with Benfica on 3 July 2018, running to 2023. Most of that contract was spent elsewhere: after loans to Espanyol and Nottingham Forest, he was briefly in the Benfica squad at the start of 2020/21 before being loaned to Reading on 5 October 2020 for the rest of that season. He returned in the summer of 2021 but was sold to Vitória de Guimarães on 29 July 2021, so he did not feature for Benfica in 2021/22.

He subsequently moved to Saudi Arabia, playing for Al Tai, NEOM and Al Fayha. He is a Guinea-Bissau international.