In 2016 he signed for Barcelona for €30 million, winning two Copa del Rey trophies and the 2017–18 La Liga during his spell as a reserve to Luis Suárez before leaving for Borussia Dortmund on loan.

On 11 November 2010, Alcácer appeared in his first match with the first team, playing the full 90 minutes in a 4–1 home win against UD Logroñés – 7–1 on aggregate – in the Copa del Rey (the fixture was also Isco's senior bow).

[5][6] On 12 August 2011, after Alcácer had scored the third and final goal in a 3–0 friendly win over Roma and was leaving the Mestalla Stadium accompanied by his parents, his father fell to the ground after suffering a heart attack; despite 30 minutes of efforts by medics to revive him, the 44-year-old died.

[15] He first found the net for Los Che in the domestic league on 25 January 2014 in a 2–2 home draw with Espanyol,[16] and grabbed another the following matchday, scoring the 3–2 winner at the Camp Nou against Barcelona.

[17] On 10 April 2014, Alcácer scored the first hat-trick of his professional career, helping overturn a 3–0 first-leg deficit to beat Basel 5–0 at home and qualify for the semi-finals of the Europa League.

[22] On 17 August 2014, Alcácer scored the first goal in a 2–1 home victory over Milan for the Orange Trophy, through a long-range shot,[23] and was assigned the number ′9′ jersey for 2014–15, previously worn by Hélder Postiga.

[29] On 30 August 2016, Alcácer signed for Barcelona for €30 million on a five-year deal,[30] and on the same day Munir El Haddadi went in the other direction on loan, to be replaced by the former as the team's fourth-choice forward behind Lionel Messi, Neymar and Luis Suárez.

[44][45] Alcácer finished 2018–19 with 18 league goals, trailing only Bayern Munich's Robert Lewandowski for top scorer, but lost his place the following January after the signing of Erling Haaland.

[49] Alcácer netted six times for the 2020–21 Europa League winners, playing the last 13 minutes of the second half and converting his attempt in the penalty shootout victory over Manchester United in the final.

[54] On 21 October, his free kick was the only goal of the final of the UAE President's Cup – delayed for six months after the death of Sheikh Khalifa – against Al Wahda, ending his team's 19-year drought in the tournament.

[61] Profiting from Costa's injury, four days later he made his first start, against Macedonia at the Estadi Ciutat de València, scoring his team's second goal in a 5–1 victory for the UEFA Euro 2016 qualifiers and also providing the assist for Sergio Busquets' third.

[66] In October 2018, amidst a good start to his spell at Dortmund and more than two years after his last international appearance, Alcácer was called back by new manager Luis Enrique for the games against Wales and England.