Coutinho scored and assisted in a dazzling 55-minute display at the Hong Kong Stadium.
Liverpool swiftly levelled as Coutinho's exquisite lofted pass allowed Mohamed Salah to nod in his second goal since joining the club.
Coutinho struck the winner two minutes before half-time, the Brazilian's thunderous effort from 18 yards flying into the top corner.
The Reds join Chelsea (x2), Bolton Wanderers, Portsmouth, Tottenham Hotspur, Manchester City and Arsenal in winning the biennial tournament.
Crystal Palace beat West Bromwich Albion in the third-place play-off earlier today.