A threshing machine or a thresher is a piece of farm equipment that threshes grain, that is, it removes the seeds from the stalks and husks. It does so by beating the plant to make the seeds fall out.

Before such machines were developed, threshing was done by hand with flails: such hand threshing was very laborious and time-consuming, taking about one-quarter of agricultural labour by the 18th century. Mechanization of this process removed a substantial amount of drudgery from farm labour. The first threshing machine was invented circa 1786 by the Scottish engineer Andrew Meikle, and the subsequent adoption of such machines was one of the earlier examples of the mechanization of agriculture. During the 19th century, threshers and mechanical reapers and reaper-binders gradually became widespread and made grain production much less laborious.

Michael Stirling is said to have invented a rotary threshing machine in 1758 which for forty years was used to process all the corn on his farm at Gateside. No published works have yet been found, but his son William made a sworn statement to his minister to this fact. He also gave him the details of his father's death in 1796.

Separate reaper-binders and threshers have largely been replaced by machines that combine all of their functions, that is combine harvesters or combines. However, the simpler machines remain important as appropriate technology in low-capital farming contexts, both in developing countries and in developed countries on small farms that strive for especially high levels of self-sufficiency. For example, pedal-powered threshers are a low-cost option, and some Amish sects use horse-drawn binders and old-style threshers.

As the verb thresh is cognate with the verb thrash (and synonymous in the grain-beating sense), the names thrashing machine and thrasher are (less common) alternate forms.