In particle physics, flavor is a property of a fermion that identifies it, a label that specifies the name of the particle.

According to the Standard Model, quarks exist in six flavors: up, down, strange, charm, bottom and top (indicated with the symbols u, d, s, c, b and t). Leptons occur in six other flavors: electron, muon, tau, electron neutrino, muon neutrino and tau neutrino (e⁻, μ⁻, τ⁻, ν_e, ν_μ and ν_τ).

Antiparticles have −1 unit of the corresponding flavor. For example, the strange and antistrange quarks (s and ) have a strangeness of −1, and +1 respectively.

A fermion of a given flavor is an eigenstate of the weak interaction; it will interact in a definite way with the W⁺, W^- and Z bosons. On the other hand, a fermion of a fixed mass (an eigenstate of the hamiltonian) is normally a superposition of various flavors, and this gives rise to processes that change the flavor. In the case of quarks, this is reflected in the so-called CKM matrix. The equivalent for neutrinos is the MNS matrix .

The term "flavor" was first applied to quarks, as in the 1970s research on hadrons had led nuclear physicists to believe there were three types of quark (the up, down and strange). The need to create a way to distinguish between the three colors of quark and three types was resolved on the way to lunch by Murray Gell-Mann and Harald Fritzsch when they passed a Baskin-Robbins advertising 31 flavors.