Two years later, various football commentators noted that Coutinho's transfer to Barcelona, was instrumental in providing the funds necessary, for Liverpool to purchase the transfer rights of both Virgil Van Dijk (from Southampton F.C. for £75 million, then a world-record transfer fee for a defender, in the 2018 January transfer window) and Alisson (from A.S. Roma for £66.9 million, then a world-record transfer fee for a goalkeeper, in the 2018 summer transfer window). Virgil Van Dijk and Alisson have both been identified as two of the most pivotal players in Liverpool's successes of winning the 2018-19 UEFA Champions League as well as the 2019-20 Premier League.
On 6 January 2018, Liverpool confirmed they had reached an agreement with La Liga club Barcelona for the transfer of Coutinho. His transfer fee was reported to be an initial £105 million, which could rise to £142 million with various clauses being met. During his medical, he was diagnosed with a thigh injury sustained a few days before joining Barcelona, ruling him out for three weeks.
He made his debut for Barcelona on 25 January 2018, in a 2–0 (2–1 on aggregate) win against his former club Espanyol in the quarter-finals of the Copa del Rey, coming on as a 68th-minute substitute for captain Andrés Iniesta. Assisted by his former Liverpool teammate Luis Suárez, Coutinho scored his first Barcelona goal on 8 February 2018 against Valencia in the second leg of the Copa del Rey semi-finals just four minutes after coming on as a half-time substitute.
On 28 October 2018, Coutinho scored a curtain-raising goal, his first Clásico goal, in a 5–1 win against Real Madrid. He scored his first Champions League goal with Barcelona on 4 October 2018 in a 4–2 away win against Tottenham Hotspur. On 16 April 2019, Coutinho scored a stunning goal in a 3–0 win over Manchester United in the Champions League quarter-finals, then he celebrated by putting his fingers in his ears and closing his eyes in response to recent criticism from fans. Former Barcelona player Rivaldo commented on the incident, by saying: "It was not a good gesture. He scored a great goal, but it's never good to do something like that to the fans", then he added, "The supporters criticise him because they know he has to give more. I don't know what happened to him during the week, what he read in the newspapers or saw on television, but you have to keep working, think about the next game and score goals. If he scores everything will change and he will be a player for the club."
In May 2018, Coutinho was named in the squad for the 2018 FIFA World Cup in Russia. On 17 June 2018, he scored in his FIFA World Cup debut with a curtain-raising, trademark right-footed curling shot from outside the penalty area into the corner of the goal in Brazil's 1–1 draw with Switzerland in their opening match at the World Cup. In the following match against Costa Rica on 22 June, he scored the opening goal in injury time of the second half and was named man of the match as Brazil won 2–0. On 6 July, he assisted Renato Augusto's goal in a 1–2 quarter-final defeat to Belgium as Brazil were eliminated from the World Cup.
In August 2018, Coutinho gained a Portuguese passport through his wife, no longer making him a non-EU player.