He took part in a brief photo session at the Nou Camp after Barcelona’s 3-0 win against Levante on Sunday.
Coutinho will officially sign his contract at 11:30 GMT on Monday before being presented to the public.
“Hi Barca fans, I am already here, it is a dream come true and I hope to see you tomorrow, ” he said in a video message on Twitter.
Later, in an interview with BarcaTV, Coutinho added he was excited to be soon playing alongside the likes of Lionel Messi and Luis Suarez.
“It is incredible to know that I am going to live with idols, players with a lot of history,” he said.
“Footballers like Leo Messi, Luis Suarez, [Andres] Iniesta, [Gerard] Pique, [Sergio] Busquets.
“I am very happy to be able to live together, to be able to learn from them and be able to win together.”
The playmaker will sign a five-and-half year deal and will have a buyout clause of 400m euros (£355m).
It is one of the most expensive transfers of all time, eclipsed only by the £200m Paris St-Germain paid Barcelona for Neymar last summer, and the £165.7m Kylian Mbappe will cost PSG at the end of his initial loan spell from Monaco.