The Spaniard was excluded from the Valencia squad that lost 1-0 at Eibar on Saturday, with coach Pako Ayesteran telling reporters, "Today he was not in the ideal situation to compete."
According to Barcelona-based newspaper Mundo Deportivo, Alcacer will sign a five-year contract later on Monday before reporting for international duty with Spain at their Las Rozas training camp in Madrid.
Alcacer will be unveiled by the club at a news conference when he returns to Barcelona after Spain open their 2018 World Cup qualifying campaign against Liechtenstein on Sept. 5.
Alcacer, who turns 23 on Tuesday, came through the ranks at Valencia and made his debut in 2010, although he spent the 2012-13 season on loan at Getafe.
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He scored 25 La Liga goals in 51 starts in the last two seasons at Valencia and was Spain's top scorer in their Euro 2016 qualifying campaign with five goals but excluded from the squad for the final tournament.
The signing of Alcacer signals an end to a summer-long search by the La Liga champions to supplement their feted frontline of Lionel Messi, Luis Suarez and Neymar, who netted a combined 131 goals in all competitions last season.
Barca had hoped to sign Kevin Gameiro from Sevilla before they were beaten to the Frenchman's signature by Atletico Madrid, and failed to land Argentine forward Luciano Vietto, who joined Sevilla on loan instead.
Should the move go through, Alcacer will become the fourth Barcelona player to join from Valencia in the last four years.
Defender Jordi Alba made the move in 2012, Jeremy Matheiu signed in 2014 and attacking midfielder Andre Gomes, a Euro 2016 winner with Portugal, joined Barcelona in July for 35 million euros.
(Editing by John O'Brien)
