Malcom was due to join Roma from Bordeaux and a flight had been booked for the 21-year-old to fly to the Italian capital for a medical.
But Barcelona swooped in with a last-ditch bid of £36million and the player sealed a switch to Nou Camp on Tuesday.
Roma are looking into taking legal action over the failed move.
And after being officially unveiled by the Catalan giants on Thursday in Portland, Malcom said: “I always wanted to go to Barcelona.
“I was always calm. My agents have been in charge of this whole issue.
“It was an opportunity to play for Barcelona and here I will play with the best players in the world.
“I am very happy to be able to be at a team like this.
“It’s very difficult to come to Barcelona, but I am going to give all I have to do a good job.
“I am very happy in my heart, I am realising a dream that I have had since childhood.”
Malcom’s team-mates take on Tottenham in their International Champions Cup game in the early hours of Sunday morning (4.05am UK time).