A scythe ( SYDHE) is an agricultural hand tool for mowing grass or harvesting crops. It is historically used to cut down or reap edible grains, before the process of threshing. The scythe has been largely replaced by horse-drawn and then tractor machinery, but is still used in some areas of Europe and Asia. Reapers are bladed machines that automate the cutting of the scythe, and sometimes subsequent steps in preparing the grain or the straw or hay.

The word "scythe" derives from Old English siðe. In Middle English and later, it was usually spelt sithe or sythe. However, in the 15th century some writers began to use the sc- spelling as they thought (wrongly) the word was related to the Latin scindere (meaning "to cut"). Nevertheless, the sithe spelling lingered and notably appears in Noah Webster's dictionaries.

A scythe consists of a shaft about 170 centimetres (67 in) long called a snaith, snath, snathe or sned, traditionally made of wood but now sometimes metal. Simple snaiths are straight with offset handles, others have an "S" curve or are steam bent in three dimensions to place the handles in an ergonomic configuration but close to the shaft. The snaith has either one or two short handles at right angles to it, usually one near the upper end and always another roughly in the middle. The handles are usually adjustable to suit the user. A curved, steel blade between 60 to 90 centimetres (24 to 35 in) long is mounted at the lower end at 90°, or less, to the snaith. Scythes almost always have the blade projecting from the left side of the snaith when in use, with the edge towards the mower; left-handed scythes are made but cannot be used together with right-handed scythes as the left-handed mower would be mowing in the opposite direction and could not mow in a team.