Polyatomic ions are groups of atoms covalently bonded together that carry a net charge. They can be considered to behave as a single unit and are often useful in the context of acid-base chemistry and in the formation of salts. Polyatomic ions can be either positively charged (cation) or negatively charged (anion). The prefix "poly-" means many in Greek, but even ions of two atoms are commonly described as polyatomic.

Examples of polyatomic ions include the hydroxide ion (OH−) and the ammonium ion (NH+4). Other examples of common polyatomic ions include:

• Phosphite (PO3−3)
• Pyrophosphite (P2O4−5)
• Phosphate (PO3−4)
• Pyrophosphate (P2O4−7)
• Arsenate (AsO3−4)
• Pyroarsenate (As2O4−7)
• Chromate (CrO2−4)
• Dichromate (Cr2O2−7)
• Carbonate (CO2−3)
• Dicarbonate (C2O2−5)

Polyatomic ions are named ending in -ate or -ite, and the majority of them are anions/07%3A_Chemical_Nomenclature/7.09%3A_Polyatomic_Ions). When drawing Lewis dot structures, the overall charge on a polyatomic ion is equal to the sum of the formal charges on each atom in the ion.