In supersymmetry, the neutralino is a hypothetical particle. In the Minimal Supersymmetric Standard Model (MSSM), a popular model of realization of supersymmetry at a low energy, there are four neutralinos that are fermions and are electrically neutral, the lightest of which is stable in an R-parity conserved scenario of MSSM. They are typically labeled N͂⁰
₁ (the lightest), N͂⁰
₂, N͂⁰
₃ and N͂⁰
₄ (the heaviest) although sometimes ${\displaystyle {\tilde {\chi }}_{1}^{0},\ldots ,{\tilde {\chi }}_{4}^{0}}$ is also used when ${\displaystyle {\tilde {\chi }}_{i}^{\pm }}$ is used to refer to charginos.

These four states are composites of the bino and the neutral wino (which are the neutral electroweak gauginos), and the neutral higgsinos. As the neutralinos are Majorana fermions, each of them is identical to its antiparticle.

If they exist, these particles would only interact with the weak vector bosons, so they would not be directly produced at hadron colliders in copious numbers. They would primarily appear as particles in cascade decays (decays that happen in multiple steps) of heavier particles usually originating from colored supersymmetric particles such as squarks or gluinos.

In R-parity conserving models, the lightest neutralino is stable and all supersymmetric cascade-decays end up decaying into this particle which leaves the detector unseen and its existence can only be inferred by looking for unbalanced momentum in a detector.

The heavier neutralinos typically decay through a neutral Z boson to a lighter neutralino or through a charged W boson to a light chargino:

The mass splittings between the different neutralinos will dictate which patterns of decays are allowed.

Up to present, neutralinos have never been observed or detected in an experiment.

In supersymmetry models, all Standard Model particles have partner particles with the same quantum numbers except for the quantum number spin, which differs by 1⁄2 from its partner particle. Since the superpartners of the Z boson (zino), the photon (photino) and the neutral higgs (higgsino) have the same quantum numbers, they can mix to form four eigenstates of the mass operator called "neutralinos". In many models the lightest of the four neutralinos turns out to be the lightest supersymmetric particle (LSP), though other particles may also take on this role.