Barcelona were unchanged from the win over SD Huesca in their last outing, as captain Messi made a landmark 768th appearance for the Spanish heavyweights.
Sociedad were first to threaten midway in the first half through Alexander Isak, before goalkeeper Alex Remiro tipped Sergio Dest's shot onto the crossbar at the other end.
The Blaugrana bagged the opening goal within a minute after Antoine Griezmann arrived in the area to riffle home a loose ball on the rebound after Sociedad failed to clear their lines.
Sociedad were breached again and conceded a second as Dest pulled wide to latch onto a through pass from Messi moments towards halftime to nestle the far corner.
Dest started the second half just as the first ended and within 53 minutes bagged a brace with a tap-in from point-blank range after meeting a cutback from Jordi Alba.
A fourth goal arrived three minutes later when Messi ran onto a first-time pass over the top from Sergio Busquets to find the top corner of the net.
Barcelona introduced fresh faces from the bench midway in the second half in Ronald Araujo and Ilaix Moriba. Soon after, the latter made an impact inside the last 20 minutes when he played a pass to Ousmane Dembele to go at the defence and fire past Remiro for the fifth of the day.
Sociedad pulled a goal back five minutes later through a thumping top-corner finish from inside the area by Ander Barrenetxea.
However, Messi had the final say two minutes from time after the Argentine turned home Alba's cross to cap off a fine evening.
