We report on a new polymorph of silver antimonate AgSbO3 discovered with the use of high-pressure high-temperature synthesis at 16 GPa and 1380 °C. The crystal structure is determined from X-ray powder diffraction, and we find this new high-pressure phase crystallizes in monoclinic space group C2/c with the following values: a = 8.4570(3) Å, b = 9.8752(3) Å, c = 8.9291(3) Å, β = 91.1750(12)°, and V = 745.56(4) Å3. We synthesized the high-pressure (16 GPa) AgSbO3 phase from the ilmenite phase as a precursor. This high-pressure monoclinic AgSbO3 consists of a three-dimensional network of corner- and edge-sharing SbO6 octahedra with channels along the c-direction containing Ag atoms. We also synthesize AgSbO3 in the defect pyrochlore phase at 4 GPa from the same ilmenite precursor and compare the Raman spectra and the cation-anion bonding of all three AgSbO3 phases. The absence of a cubic perovskite form of AgSbO3 even at pressures of ≤16 GPa is likely due to the covalency of the Sb-O bonds and the moderate electronegativity of Ag+. Hybridization of Ag d and O p orbitals results in a variation of Ag-O distances that correlates with the band gap, which is in qualitative agreement with the density of states around the Fermi level from our density functional calculations. We compare AgSbO3 with other ABX3 compounds to elucidate the dependence of the structure on the constituent atoms.