Orthonitrate is a tetrahedral anion of nitrogen with the formula NO3−4.

The corresponding oxoacid, orthonitric acid (H3NO4), is hypothetical and has never been observed.

Sodium and potassium orthonitrate can be prepared by fusion of the nitrate and metal oxide under high temperatures[2] and ideally high pressures (several GPa).

[3] The resulting orthonitrates are white solids which are extremely sensitive to moisture and CO2, decomposing within minutes to hydroxides, carbonates, and nitrates upon exposure to air.

[1] The orthonitrate ion is tetrahedral with N–O bond lengths of 139 pm, which is unexpectedly short, indicating that polar interactions are shortening the bond.