The 25-year-old Brazilian signed a contract alongside Barcelona president Josep Maria Bartomeu, before performing keepy-uppies in front of supporters.
"I want to thank the president and everyone who has made it possible," said Coutinho.
"I am very happy, it's a dream come true and I hope to be up to the job on the field."
Bartomeu added: "All the Barca fans are excited to have Coutinho here.
"It hasn't been easy but his effort in coming, along with his strength and patience, have been key."
Coutinho passed his medical, but it did show he has a right thigh injury.