Hammer mill is a machine that can crush or shred various materials into smaller particles. It works by using a series of small hammers that rotate at high speed in a steel drum, repeatedly hitting the materials until they are reduced in size. This machine has a wide range of applications in many industrial fields, such as:

- Ethanol plants (grains)

- Farm machinery, grinding grains into coarse flour for livestock

- Cellulose pulp

- Juice production

- Grinding used pallets into wood chips

- Milling grains

- Livestock and aquaculture feed production

- Sawmills, compressing cutting waste and shavings into boiler fuel or wood chips

- Laboratory scale hammer mills, used for preparing culture media in life sciences

- Shredding paper

- Shredding cars (see car shredder residue)

- Shredding yard and garden waste for composting

- Crushing large stones

- Waste management

- High speed industrial hammer mills, used for waste fragmentation

- Low speed horizontal pallet mills

Hammer mill can adjust the number, shape, size and speed of the hammers, as well as the aperture and position of the screen, according to different purposes and material characteristics, to achieve different particle size distribution and output. hammer mill can be used as primary, secondary or tertiary crushers. Small hammer mills can be powered by household electricity. Large hammer mills used in automobile shredders may require diesel or electric motors ranging from 2000 to over 5000 horsepower (1.5 - 3.7MW). Screenless hammer mills use air flow to separate small particles from larger ones. They are designed to be more reliable, and also claim to be much cheaper and more energy efficient than regular hammer mills. The design and structure of hammer mills are always determined by the final use.

Hammer mill was first invented by Chinese mathematician and engineer Zu Chongzhi in 488 AD, and was tested by Emperor Wu of Southern Qi (r. 482–493). The water-powered trip hammer was driven by a small mountain stream connected to a lower river.