Coutinho, who has been linked with a move to Barcelona, will reportedly earn £150,000 a week as of July, making him the highest paid player at Anfield.
"It is a club that I am very grateful to and this shows my happiness here," he told the club's website.
"I signed this new contract to stay here for a few more years because it's a great honour for me.
"It gives me great happiness because I was welcomed here with open arms by everyone at the club and the supporters right from my first day."
The 24-year-old has scored 34 goals in 163 appearances for the Reds since his £8.5m signing from Inter Milan in January 2013.