I know everyone thinks it's science fiction. It's cited as an early entry in the genre. H.G. Wells, it is said, is the father of modern science fiction.
(I'd point more at Mary Shelley as the mother of the genre, but Wells certainly does fit the bill, through some of his other works.)
The thing is, though... This is barely a science fiction book. If anything, it's a class-conscious satire, a Utopian work in a the same genre as Swift's Gulliver's Travels, but using a scientific framework based on then-current understandings of the world to hold it up and make its point.
