Using density functional theory+Hubbard U (DFT+U) calculations, we investigate the spin states and nuclear hyperfine interactions of iron incorporated in magnesium silicate (MgSiO 3) post-perovskite (Ppv), a major mineral phase in the Earth's D" layer, where the pressure ranges from ~120 to 135GPa. In this pressure range, ferrous iron (Fe 2+) substituting for magnesium at the dodecahedral (A) site remains in the high-spin (HS) state; intermediate-spin (IS) and low-spin (LS) states are highly unfavorable. As to ferric iron (Fe 3+), which substitutes magnesium at the A site and silicon at the octahedral (B) site to form (Mg,Fe)(Si,Fe)O 3 Ppv, we find the combination of HS Fe 3+ at the A site and LS Fe 3+ at the B site the most favorable. Neither A-site nor B-site Fe 3+ undergoes a spin-state crossover in the D" pressure range. The computed iron quadrupole splittings are consistent with those observed in Mössbauer spectra. The effects of Fe 2+ and Fe 3+ on the equation of state of Ppv are found nearly identical, expanding the unit cell volume while barely affecting the bulk modulus.