Ana Botella, the mayor of Madrid with Spain's conservative Political Party, made the comments during a press conference welcoming the city's new batch of firefighters.
"I'm a firm believer in the equality of women," said Botella, when asked why all of the 214 firefighters were men.
She then said, however that the job required "special physical training".
"And the physical conditions of men and women, because of reasons of biology, make them different — we are different — and that's why there aren't any women among the 214 firefighters," Botella concluded.
"Once again, Ana Botella makes absolutely inappropriate comments" tweeted Jaime Lissavetzky, spokesman for the Socialist Party opposition on Madrid's city council, adding "Madrid doesn't deserve this".
Botella is infamous in Spain for her ability to put her foot in it.
She featured in Time's 2013 The Year in Mayors' Gaffes list which graded ten poor-performing global city officials from "bad to worst".
She made the list for having "jumbled several answers during a September press conference about her city's (failed) bid for the 2020 Olympics", said the US magazine.
The speech - and her struggle with English - was panned by local media," the magazine's editors added.
That speech contained the now infamous line: "There is nothing quite like a relaxing cup of café con leche (coffee with milk) in Plaza Mayor".