A hay knife is an agricultural hand tool: a long-bladed knife which may have large rounded serrations on the edge, or a smooth edge used for sawing off sections at the end of a stack or compact pile of hay or silage. In the south of England hay knives may have smooth edges.^{[citation needed]}

Hay knives are needed as loose hay or silage becomes compacted within a stack: to remove it a hay knife is used to make a vertical cut so that sections can be removed easily as the intertwined stalks are cut. The offset handle allows the user to work down a face. Once one section has been removed the worker starts again at the top creating another section to be removed.