The structure and hydration energy of small water clusters containing an ammonium ion are investigated within density functional theory by using the Becke-Lee-Yang-Parr (BLYP) generalized gradient corrections to the local density approximation to the exchange and correlation energy, norm-conserving pseudopotentials, and a plane-wave expansion of Kohn-Sham orbitals. The calculated hydration energies are in good agreement with experimental data and with available quantum chemical calculations. The result of this study furnishes us with further evidence of the ability of the BLYP functional to provide a good description of hydrated systems.