After two semi-finals and a final which took place in February and March 2021, "Love Is on My Side" performed by the Black Mamba emerged as the winner after achieving the joint highest score following the combination of votes from seven regional juries and a public televote, winning the tiebreaker due to having more points from the latter.

Performing during the show in position 12, "Love Is on My Side" was announced among the top 10 entries of the second semi-final and therefore qualified to compete in the final on 22 May.

[1] Portugal had won the contest on one occasion: in 2017 with the song "Amar pelos dois" performed by Salvador Sobral.

Portugal's least successful result has been last place, which they have achieved on four occasions, most recently in 2018 with the song "O jardim" performed by Cláudia Pascoal.

Twenty entries competed in the competition that consisted of two semi-finals held on 20 and 27 February 2021 leading to a ten-song final on 6 March 2021.

[5] Results during the semi-finals were determined by the 50/50 combination of votes from a jury panel appointed by RTP and public televoting, while results during the final were determined by the 50/50 combination of votes from seven regional juries and public televoting, which was opened following the second semi-final and closed during the final show.

[8] The composers, which both created the songs and selected its performers, were required to submit the demo and final versions of their entries by 30 November and 31 December 2020, respectively.

In each semi-final eight entries competed and four advanced to the final based on the 50/50 combination of votes of a jury panel consisting of Marta Carvalho, NBC, Paulo de Carvalho, Rita Carmo, Rita Guerra and Vanessa Augusto, and a public televote.

The ten entries that qualified from the two preceding semi-finals competed and the winner, "Love Is on My Side" performed by the Black Mamba, was selected based on the 50/50 combination of votes of seven regional juries and a public televote.

[13] Ricardo Ribeiro, Ana Moura, Camané, Dino D'Santiago, Portuguese Eurovision 2018 entrant Cláudia Pascoal together with Clã, Filipe Sambado and Sérgio Godinho, and Elisa performed as the interval acts.

[14][15] According to Eurovision rules, all nations with the exceptions of the host country and the "Big Five" (France, Germany, Italy, Spain and the United Kingdom) are required to qualify from one of two semi-finals in order to compete for the final; the top ten countries from each semi-final progress to the final.

This jury judged each entry based on: vocal capacity; the stage performance; the song's composition and originality; and the overall impression by the act.

[23] The individual rankings of each jury member in an anonymised form as well as the nation's televoting results were released shortly after the grand final.