Guano (Spanish from Quechua: wanu) is the accumulated excrement of seabirds or bats. Guano is a highly effective fertiliser due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. Guano was also, to a lesser extent, sought for the production of gunpowder and other explosive materials.

The 19th-century seabird guano trade played a pivotal role in the development of modern input-intensive farming. The demand for guano spurred the human colonisation of remote bird islands in many parts of the world.

Unsustainable seabird guano mining processes can result in permanent habitat destruction and the loss of millions of seabirds.

Bat guano is found in caves throughout the world. Many cave ecosystems are wholly dependent on bats to provide nutrients via their guano which supports bacteria, fungi, invertebrates, and vertebrates. The loss of bats from a cave can result in the extinction of species that rely on their guano. Unsustainable harvesting of bat guano may cause bats to abandon their roost.

Demand for guano rapidly declined after 1910 with the development of the Haber–Bosch process for extracting nitrogen from the atmosphere.

Guano mining continues in Chile with the annual guano production in Chile ranging from 2,091 to 4,601 metric tons per year in the 2014–2023 period.

Seabird guano is the fecal excrement from marine birds and has an organic matter content greater than 40%, and is a source of nitrogen (N) and available phosphate (P₂O₅). Unlike most mammals, birds do not excrete urea, but uric acid, so that the amount of nitrogen per volume is much higher than in other animal excrement.

Seabird guano contains plant nutrients including nitrogen, phosphorus, calcium and potassium.