The Danish global has inked a four-year deal at Camp Nou, after the Catalan giants were handed special dispensation to sign him outside of the transfer window.
La Liga gave Barcelona permission to make a signing outside the transfer window after Ousmane Dembele was ruled out for six months with an injury.
"FC Barcelona have paid the buy out clause of the player Martin Braithwaite and therefore he is no longer contractually tied to Club Deportivo Leganés", the club statement read.
The club made the announcement via their official social media page on Wednesday, February 20, bringing a sigh of relief to anxious fans about the Spanish champions' lack of attacking options.
Dembele was ruled out for six months after surgery earlier this month and, unlike in other major European leagues, clubs in Spain are allowed to sign a player outside the transfer window under special circumstances. "We want to raise our voices because we're in a very serious situation and have suffered a lot of damage", he told reporters.
"Barca have acted in correct manner".
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"We believe this rule is not fair and Barcelona have benefited from it while Leganes have been harmed by it".
The La Liga champions paid the £15m buyout clause for the Denmark global, who scored eight goals in 20 matches for Leganes this season.
Braithwaite previously played for Denmark's Esbjerg, French clubs Toulouse and Bordeaux, and England's Middlesbrough.
"We cannot understand the current regulation that a club, having a long-term injury, can perform this operation unilaterally and transfer their problem to our club".
"We are excited about the possibility that the FA will allow us to sign a player".
He has made 39 appearances for Denmark, including four matches at the 2018 World Cup.