Due to limits on players without European Union passports, Malcom was left out from the Barcelona squad that won the 2018 Supercopa de España. On 25 August, he made his official debut for the club in a 1–0 victory over Real Valladolid at the José Zorrilla, replacing compatriot Philippe Coutinho with six minutes left. On 6 November, Malcom scored his first competitive goal for Barcelona in a 1–1 away draw against Inter Milan in the UEFA Champions League. On 6 February 2019, Malcom was given a surprising start against Real Madrid in the Copa del Rey semi-finals first leg, scoring the equalizer in a 1–1 draw. On 2 April 2019, Malcom scored his first La Liga goal in a 4–4 draw away against Villarreal CF.