That depends on the lens. Sigma makes their lenses with a variety of mounts. If the lens has a Nikon mount then you don't need an adapter at all.
Since the Nikon lens mount has a deeper flange depth than other mounts, you cannot fit a lens with another mount onto a Nikon body without losing the ability to focus to infinity. Because of this, you won't find an adapter for another mount. Adapters are available to fit Nikon lenses onto other cameras but not the other way around.
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That depends on the lens. Sigma makes their lenses with a variety of mounts. If the lens has a Nikon mount then you don't need an adapter at all.
Since the Nikon lens mount has a deeper flange depth than other mounts, you cannot fit a lens with another mount onto a Nikon body without losing the ability to focus to infinity. Because of this, you won't find an adapter for another mount. Adapters are available to fit Nikon lenses onto other cameras but not the other way around.