Braithwaite, 28, has signed until 2024 and could make his debut on Saturday against Eibar after Barcelona paid his €18 million release clause. His new release clause at Barcelona is €300m, while Leganes, battling to avoid relegation in La Liga, will not be able to sign a replacement.
Lionel Messi has said that Barcelona is "my home" amid speculation that he could make a move away from the Camp Nou, while adding that the club shouldn't be viewed as favorites for the Champions League.
There have been suggestions that Messi has been frustrated on and off the pitch this season, but he has appeared to commit his future to the Catalans.
"I love Barcelona although I really do miss Rosario," he told Mundo Deportivo. "This is my home, I've been here longer than Argentina.