砷酸为砷(V)的含氧酸，分子式H₃AsO₄。三元中强酸，酸性及其他性质均类似磷酸。

由五氧化二砷溶于水而得。

反应也可向左进行，如加入五氧化二磷作吸水剂时，又可从砷酸产生五氧化二砷。

砷酸也可由砷或三氧化二砷被浓硝酸氧化而得。该反应会生成副产物三氧化二氮。反应后于29.5°C以下浓缩，可析出砷酸半水合物细小板状结晶。于29.5℃以上，则析出三砷酸（H₅As₃O₁₀）。

反应结束后，若将所得液体冷却，可得到以无色结晶形式析出的砷酸的半水合物H₃AsO₄·½H₂O。如果进一步降低温度的话，还可以获得砷酸的二水合物H₃AsO₄·2H₂O。

Arsenic acid is the chemical compound with the formula H₃AsO₄. More descriptively written as AsO(OH)₃, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but only found in solution where it is largely ionized. Its hemihydrate form (H₃AsO₄·½H₂O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.