The visitors were leading thanks to Fabian Orellana's 83rd-minute header when the referee halted play as a smoking object landed just outside the Celta penalty area.
Upon landing on the pitch, the canister caught fire and subsequently left scorch marks on the grass as smoke billowed out from both ends.
Fans were instructed to leave the stadium as supporters and players alike rushed to escape the smoke, covering their eyes in the process.
Play resumed almost 25 minutes later after the object was cleared and the players re-entered the pitch to play the final few minutes, with Celta subsequently scoring a second to win 2-0.
The incident is the second in the space of a few days to blight Spanish football after Atletico Madrid were fined €600 following an incident in which a fan threw a cigarette lighter at Cristiano Ronaldo during their Copa del Rey clash at Vicente Calderon on Tuesday.
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