We suggest a real right-handed sneutrino, Ñ1, as a good dark matter candidate in a supersymmetric Z' model realizing the seesaw mechanism. When the extra gaugino, Z̃', is lighter than Z', the thermal freeze-out of the dark matter annihilation to righthanded neutrinos, Ñ1 Ñ1 ? NN, through the t-channel Z̃' exchange is shown to produce the right dark matter density. It is essential to include the decay and inverse decay of N in this process, otherwise N decouples too early and thus dark matter is overproduced. At the LHC, the search for the seesaw mechanism can be made by observing the signatures of pp ? Z̃' Z̃' → NN+ 6pT as Z̃' can be copiously produced from the cascade decays of gluinos/squarks, which is complementary to the search of pp → Z' → NN. This may also open up a promising new channel of finding the Higgs boson from the displaced N decay. © SISSA 2011.