An acidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor of hydroxide ions effectively functioning as a Lewis acid. Acidic oxides will typically have a low pK_a and may be inorganic or organic. A commonly encountered acidic oxide, carbon dioxide produces an acidic solution (and the generation of carbonic acid) when dissolved. Generally non-metallic oxides are acidic.

The acidity of an oxide can be reasonably assumed by its accompanying constituents. Less electronegative elements tend to form basic oxides such as sodium oxide and magnesium oxide, whereas more electronegative elements tend to produce acidic oxides such as carbon dioxide and phosphorus pentoxide. Some oxides, like aluminium oxides, are amphoteric, while some oxides may be neutral.

Acidic oxides are of environmental concern. Sulfur and nitrogen oxides are considered air pollutants as they react with atmospheric water vapour to produce acid rain.

Carbonic acid is an illustrative example of the Lewis acidity of an acidic oxide.

This property is a key reason for keeping alkali chemicals well sealed from the atmosphere, as long-term exposure to carbon dioxide in the air can degrade the material.

Aluminium oxide (Al₂O₃) is an amphoteric oxide; it can act as a base or acid. For example, with base different aluminate salts will be formed:

Silicon dioxide is an acidic oxide. It will react with strong bases to form silicate salts.

Silicon dioxide is the anhydride of silicic acid: