海鸟粪（Guano）又称鸟粪石，是海鸟、蝙蝠或海豹排泄物与动植物残体混杂而成的一种细颗粒混合物。它可作肥料，而且在历史上也曾是氮、磷元素的重要来源，用于战争用火药的生产之中。

从成分上来讲，海鸟粪是多种钙磷酸盐的混合物，包括钙磷矿（CaHPO₄ · 2 H₂O）、三斜磷钙矿（CaHPO₄）、白磷钙矿（Ca₉(Mg,Fe)[HPO₄/(PO₄)₆]）以及各类磷灰石、硝酸盐和有机物等。根据氮磷的含量不同，又可分为氮质海鸟粪与磷质海鸟粪两种，其中磷质海鸟粪因含不溶磷酸盐，常发现于多雨、氮成分流失较多地区。

Guano (via Spanish, ultimately from the Quechua wanu) is the excrement of seabirds, cave-dwelling bats, pinnipeds, or (in English usage) birds in general.<ref name=""h2g2">"Seabird Guano and Bat Dung". 2004-11-25. Retrieved 2013-05-11. </ref> As a manure, guano is a highly effective fertilizer due to its exceptionally high content of nitrogen, phosphate and potassium: nutrients essential for plant growth. The 19th-century guano trade played a pivotal role in the development of modern input-intensive farming practices and inspired the formal human colonization of remote bird islands in many parts of the world. During the twentieth century, guano-producing birds became an important target of conservation programs and influenced the development of environmental consciousness. Today, guano is increasingly sought after by organic farmers.