Is a Sigma lens compatible with a Nikon D50?

I have a Nikon D50 and was just given a Sigma ZOOM MASTER 1:3.5~4.5 35-70mm Multi-Coated lens. I was told that the lens would fit on my D50 body, but in attempting to attach it… Well, it doesn't.

Is it meant to? Do I need to get something to fit it on there? Or is it not compatible at all?

Only if it has the Nikon mount, Sigma make lenses for a few cameras, all the mounts are different. It might even be an old Nikon mount.

Maybe its mount is not for Nikon. Then you have to buy a adapter to fit it to your camera. But first you have to know which mount does your lens have.

A lens with that specification is quite old, possibly over 30 years old, maybe even over 40 years, thus it may not have all the gismos needed to fit it safely to a modern Nikon DSLR. If it does not fit easily then don't force it. It is too old to be worth modifying to fit modern Nikons. As others say, it might not even be an old Nikon mount, but possibly Minolta etc. (In which case definitely no chance)

The Nikon F mount bayonet has not changed since the original Nikon F. Nikon added electronic contacts later, but still uses the same bayonet, so in general, any F mount lens will mount on any Nikon body. There are other things that may prevent a lens from actually working properly, but the mount itself should work anyway.

It is most likely that your lens is not actually for Nikon F mount, and isn't worth much to try and make it fit.