FC Seoul insisted the mannequins - used in the absence of fans, who are banned because of the coronavirus - had "no connection to sex toys".
But some of the artificial spectators, deployed for Sunday's game against Gwangju FC, wore T-shirts with the logo of SoloS, a sex toy seller.
Other mannequins, which wore facemasks and were separated according to social distancing guidelines, held placards advertising the company and some of its models.
Most of them were female, but some were male. FC Seoul said the company which provided the mannequins had previously provided them to SoloS, hence the advertising in which they were clad.
"We are sincerely sorry for causing deep concern to fans," FC Seoul said in a statement, following uproar from fans and media. We have confirmed from the very beginning that they had no connection to sex toys," the club said.