Bog iron is a form of impure iron deposit that develops in bogs or swamps by the chemical or biochemical oxidation of iron carried in solution. In general, bog ores consist primarily of iron oxyhydroxides, commonly goethite (FeO(OH)).

Iron-bearing groundwater typically emerges as a spring and the iron in it forms ferric hydroxide upon encountering the oxidizing environment of the surface. Bog ore often combines goethite, magnetite, and vugs or stained quartz. Oxidation may occur through enzyme catalysis by iron bacteria. It is not clear whether the magnetite precipitates upon the first contact with oxygen, then oxidizes to ferric compounds, or whether the ferric compounds are reduced when exposed to anoxic conditions upon burial beneath the sediment surface and reoxidized upon exhumation at the surface.

Bog iron, like other hydrous iron oxides, has a specific affinity for heavy metals. This affinity combined with the porous structure and high specific surface area of bog iron make it a good natural sorbent. These properties combined with the fact that bog iron is cheap to obtain are incentives for its utilization in environmental protection technologies.

Iron made from bog ore will often contain residual silicates, which can form a glassy coating that imparts some resistance to rusting.