Biomagnetism describes the biological origin of magnetism within living organisms. This phenomenon occurs due to the formation of iron-based minerals that exhibit magnetic ordering at room temperature. Perhaps the most studied form of biomagnetism originates in bacteria, especially magnetotactic bacteria producing internal magnetite grains and iron-reducing bacteria producing magnetite nanoparticles externally as a byproduct of iron respiration. They likely contribute to a significant proportion of environmental magnetite. The emergence of biomagnetism remains unclear, although it is thought that magnetotactic bacteria evolved this mechanism several billion years ago. Understanding how and why microorganisms generate biomagnetism is helping to shed light on the origin of life on Earth and potentially on other planets.

Biomagnetism is also of broad interest to industrial and environmental applications.