"It's an expression of quite a bit of prejudice and maybe even fear, but it is clear that it is a negative signal in every way, there's no doubt about it," Bildt, whose country holds the current European Union presidency, told Svergies Radio (SR).
Swiss voters on Sunday approved by a majority of 57.5 percent a ban on minarets in a referendum.
Bildt said he found it odd that such a decision was put to a referendum.
"Normally Sweden and other countries have city planners that decide this kind of issue. To decide this kind of issue in a referendum seems very strange to me," he said.