Will a Nikon Sigma Fisheye EX DG 8mm 3.5 lens fit a Canon Rebel XSi?

Might be buying this one soon!

No, it's made for Nikon cameras, as the name indicates.

There most certainly also is a Canon version of it though.

But… Are you certain that you want a fisheye and not an ultra wide angle lens? Due to the extreme distortion fisheye lenses are some kind of special application lens. Odds are that the novelty effect will wear off quickly and it will end up gathering dust on your shelf.

No, not if it is made for Nikon. Nikon and Canon have different mounts, Sigma make lenses for both companies. If it says it is a Nikon mount then it will not fit your Canon Rebel XSi - you're looking for "Canon fit" or "EF" / "EF-S" mount lenses.

I guess if you got an adapter it might. But it makes more sense to buy a lens with a canon EF mount.