The 25-year-old finalised terms on a £142m deal at Camp Nou on Monday morning, bringing an end to months of drawn-out speculation.
Coutinho is now behind only Neymar in the list of world's most-expensive players, with the forward setting Paris Saint-Germain back £198m in the summer, and the two players have already exchanged words.
"I play with Neymar in the national team and we have been friends since we were kids," Coutinho told reporters. "He congratulated me and told me that there are incredible changing rooms.
Coutinho is not expected to make his Barca bow until February after a thigh strain was discovered while undergoing a medical.