A zwitterion (from German "Zwitter" — "hybrid," "hermaphrodite") is a compound with acidic and basic groups in the same molecule. At neutral pH most zwitterions are therefore negatively charged anions and positively charged cations at the same time. Zwitterions usually have a high solubility in water due to their charged groups and a poor solubility in most organic solvents.

Zwitterions make good buffer solutions — they resist change to the pH of a solution by selective ionisation. In the presence of acids, zwitterions accept the hydrogen ions, removing them from the solution. In the presence of bases, zwitterions donate hydrogen ions to the solution, again balancing the pH.

Typical examples of zwitterions are:

• Amino acids with their carboxylic acid groups (-COOH) and their basic amino groups (-NH₂).
• Zwitterionic buffer compounds like HEPES, PIPES, CAPS, or MOPS.
• Natural products like the alkaloids psilocybin and lysergic acid.