I’d say he was a ‘weird’ writer if that can be described as a genre in loosely the same mold as Poe, Machen, MR James, etc…But the weird is kind of hard to define. I guess they’re kind of science fantasies, but not in the way that we usually describe them, which is usually fairly lite in tone. The ideology behind Lovecraft is definitely inspired by science but the Cthulhu mythos can hardly be said to be based in fact in any meaningful way nor could it be described as speculative – Lovecraft remains supernatural in my opinion. So if I had to lean one way or the other then I guess fantasy would have to do. I find ‘horror’ as a genre really hard to pin down. But maybe all genres are?
Either way Lovecraft is quickly becoming my fave author these days