For the 1982–83 campaign, Alonso moved to FC Barcelona, being used frequently in his first year but losing his importance after the purchase of Scotland's Steve Archibald, and never regaining it again. Still, he scored three goals against IFK Göteborg in the 1986 European Cup semi-finals, allowing the Catalans to reach the final of the competition against FC Steaua București in Seville, where he came on as a substitute in extra time, in an eventual penalty shootout loss; he was one of four players that had his attempt saved by opposing goalkeeper Helmuth Duckadam.