碳酸（英语：Carbonic acid），原来也称挥发酸（Volatile acid）和呼吸酸（Respiratory acid）， 化学式H₂CO₃，是酸的一种。二氧化碳（CO₂）溶于水后，一部分二氧化碳会与水化合，形成碳酸。该反应是一个可逆反应，方程序如下：

该反应在常温下的平衡常数是K_h=1.70×10；因此大部分二氧化碳都不会参与反应。假若没有催化剂存在，反应速率十分缓慢，其反应速率常数仅为0.039 s（正反应）以及23 s（逆反应）。

碳酸是无机物。

Not to be confused with Carbolic acid, an antiquated name for phenol.

Carbonic acid is also an archaic name for carbon dioxide.

Carbonic acid is a chemical compound with the chemical formula H₂CO₃ (equivalently OC(OH)₂). It is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H₂CO₃. In physiology, carbonic acid is described as volatile acid or respiratory acid, because it is the only acid excreted as a gas by the lungs.