Mechanochemistry (or mechanical chemistry) is the initiation of chemical reactions by mechanical phenomena. Mechanochemistry thus represents a fourth way to cause chemical reactions, complementing thermal reactions in fluids, photochemistry, and electrochemistry. Conventionally mechanochemistry focuses on the transformations of covalent bonds by mechanical force. Not covered by the topic are many phenomena: phase transitions, dynamics of biomolecules (docking, folding), and sonochemistry. Mechanochemistry also encompasses mechanophores which are molecules that undergo predictable changes in response to applied stress. Two types of mechanophores are mechanochromic ones in which a force causes a change in molecular structure and subsequently color and acid releasing mechanophores that release small amounts of an acid such as HCl in response to an applied force.

Mechanochemistry is not the same as mechanosynthesis, which refers specifically to the machine-controlled construction of complex molecular products.

In natural environments, mechanochemical reactions are frequently induced by physical processes such as earthquakes, glacier movement or hydraulic action of rivers or waves. In extreme environments such as subglacial lakes, hydrogen generated by mechnochemical reactions involving crushed silicate rocks and water can support methanogenic microbial communities. And mechanochemistry may have generated oxygen in the ancient Earth by water splitting on fractured mineral surfaces at high temperatures, potentially influencing life's origin or early evolution.

The primal mechanochemical project was to make fire by rubbing pieces of wood against each other, creating friction and hence heat, triggering combustion at the elevated temperature. Another method involves the use of flint and steel, during which a spark (a small particle of pyrophoric metal) spontaneously combusts in air, starting fire instantaneously.

Industrial mechanochemistry began with the grinding of two solid reactants. Mercuric sulfide (the mineral cinnabar) and copper metal thereby react to produce mercury and copper sulfide:

A special issue of Chemical Society Review was dedicated to mechanochemistry.

Scientists recognized that mechanochemical reactions occur in environments naturally due to various processes, and the reaction products have the potential to influence microbial communities in tectonically active regions. The field has garnered increasing attention recently as mechanochemistry has the potential to generate diverse molecules capable of supporting extremophilic microbes, influencing the early evolution of life, developing the systems necessary for the origin of life, or supporting alien life forms. The field has now inspired the initiation of a special research topic in the journal Frontiers in Geochemistry.

Earthquakes crush rocks across Earth's subsurface and on other tectonically active planets. Rivers also frequently abrade rocks, revealing fresh mineral surfaces and waves at a shore erode cliffs fracture rocks and abrade sediments.