In particle physics, strange particles are those particles that, in addition to containing up and down quarks, contain a strange quark. Strange particles were originally so named because they exhibited greatly increased lifespans in comparison with other particles.

Strange particles are particles that are only produced in pairs in electromagnetic and strong interaction processes. They possess an extra quantum number called strangeness, and this quantum number is conserved in all strong and electromagnetic scattering and decay processes. Strangeness was later related to the fact that strange particles contain a new quark, known as the strange quark.

Strangeness (and the strange quantum number) are elementary particles that are conserved by the strong force. The quantum number reflects the difference between the baryon number and the hypercharge.

By the 1960s, the number of elementary particles was increasing rapidly. Among the strange particles discovered were K mesons, K+ , K...