A T-shirt tribute to record signing Ousmane Dembele came before the match after it was revealed he’d be out for four months.
Messi found the net from the spot early on and Paulinho soon doubled Barca’s lead, heading home from a corner.
Denis Suarez added a third early in the second half after Messi had been denied.
But the Argentine superstar scored three more in the second-half to cap another sensational display.
Barcelona: Ter Stegen, Semedo, Pique, Mascherano, Digne, Busquets, Iniesta, Denis, Paulinho, Deulofeu, Messi
Eibar: Dmitrovic, Galves, Escalante, Capa, Inui, Enrich, Paulo, Dani Garcia, Arbilla, Junca, Joan Jordan