Alisson finally made his Serie A debut on the opening weekend of the 2017–18 Serie A campaign, starting in a 1–0 win over Atalanta. He then made his first appearance in the Derby della Capitale on 18 November, starting in a 2–1 win over local rivals Lazio. He was praised for his performances in the UEFA Champions League, and played a key role in the club's campaign in which they reached the semi-finals. The club didn't concede a single goal at the Stadio Olimpico in the Champions League, until the match against Liverpool in the semi-finals on 2 May 2018, which they won 4–2 on the night, but lost 7–6 on aggregate. Alisson received praise for his performances throughout the 2017–18 season. He kept a total of 22 clean sheets across the 2017–18 season, keeping 17 clean sheets in the league and 5 clean sheets in the UEFA Champions League.