Selenic acid is the chemical compound with the formula H₂SeO₄. It is an oxoacid of selenium, and its structure is more accurately described as (HO)₂SeO₂.

As predicted by VSEPR theory, the selenium center is tetrahedral, with a Se–O bond length of 161 pm. In the solid state, it crystallizes in an orthorhombic structure.

Due to the instability of selenium trioxide, it is impractical to synthesize selenic acid by dissolving selenium trioxide in water, unlike sulfuric acid, which can be synthesized by dissolving sulfur trioxide in water. Instead, it must be prepared by oxidising selenium compounds in lower oxidation states.

One method of preparing selenic acid is by the oxidation of selenium dioxide with hydrogen peroxide:

To obtain the anhydrous acid as a crystalline solid, the resulting solution is evaporated at temperatures below 140 °C (413 K; 284 °F) in a vacuum.

Selenic acid may also be prepared by the oxidation of selenous acid (H₂SeO₃) with halogens, such as chlorine or bromine, or with potassium permanganate. However, using chlorine or bromine as the oxidising agent also produces hydrochloric or hydrobromic acid as a side-product, which needs to be removed from the solution since they can reduce the selenic acid to selenous acid.