The second harmonic generation (SHG) responses of the paraelectric and ferroelectric phases of KH₂PO₄ (KDP) were calculated by first-principles density functional theory (DFT) calculations, and the individual atom contributions to the SHG responses were analyzed by the atom response theory (ART). We show that the occurrence of static polarization does not enhance the SHG responses of the ferroelectric KDP, and that the Kleinman symmetry is reasonably well obeyed for the paraelectric phase, but not for the ferroelectric phase despite that the latter has a larger bandgap. This is caused most likely by the fact that the ferroelectric phase has lower-symmetry local structures than does the paraelectric phase. The contribution to the SHG response of an individual K⁺ ion is comparable to that of an individual O²⁻ ion. The contributions of the O²⁻ and K⁺ ions arise overwhelmingly from the polarizable parts of the electronic structure, namely, from the valence bands of the O-2p nonbonding states and from the conduction bands of the K-3d states.