Negotiations between the two clubs have been underway for the past weeks, and a fee and payment schedule has been agreed.
A medical is expected to be conducted on Monday, with Coutinho expected to fly to Barcelona as soon as the deal gets closed.
The Brazil international did not travel with the squad to Dubai for a warm-weather training, and has not featured for the team since the window opened, due to a minor thigh injury.
The 25-year-old handed in a transfer request in the summer in a bid to force a move, after Liverpool rejected three bids from the Catalan club, the largest of which was worth £113 million.
Coutinho has made 20 appearances in all competitions for Liverpool this season, scoring 12 goals.
Liverpool bought Coutinho for £8.5m from Inter Milan in 2013.