The 20-year old Brazilian attacking midfielder joins the Reds on a long-term deal for a fee of around Â£8.5million after passing a medical and obtaining a work permit.
"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," he said.
"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."