The 20-year-old Brazilian became manager Brendan Rodgers' second signing of the January transfer window after finalising the details of his medical and work permit.
Coutinho is on his fourth club - all in different countries - in three years, but having spoken at length with compatriot and new team-mate Lucas Leiva, he is confident he will adapt to life on Merseyside.
"I've been talking to him for some time and he said good things about Liverpool," said the midfielder, who began his career with Vasco da Gama before moving to Inter, who loaned him to Espanyol for the latter half of last season.
"He believes this might be a nice club for me and now it's happening. I'm very confident and excited to come and start to train with the team and be available to the manager
"I've been playing in Italy for two and a half years and I believe I will adapt with ease here.
"I'm feeling very happy, it's a very important step in my career and I'm expecting to come here and play good football to give my contribution on the pitch.
"Liverpool is a great club with great players. We've always heard about Liverpool's history in Brazil.
"They are the club that have been interested in me and that have showed this interest so I know they believe in me and my football."
- Sports & Recreation