In particle physics the semileptonic decay of a hadron is a decay caused by the weak force in which one lepton (and the corresponding neutrino) is produced in addition to one or more hadrons. An example for this can be

K⁰
 → 
e⁻
 + 
ν
_e + 
π⁺

This is to be contrasted with purely hadronic decays, such as
K⁰
 → 
π⁺
 + 
π⁻
, which are also mediated by the weak force.

Semileptonic decays of neutral kaons have been used to study kaon oscillations.