Saponins (Latin sapon, 'soap' + -in, 'one of') are bitter-tasting, usually toxic plant-derived secondary metabolites. They are organic chemicals that become foamy when agitated in water and have high molecular weight. They are present in a wide range of plant species throughout the bark, leaves, stems, roots and flowers but particularly in soapwort (genus Saponaria), a flowering plant, the soapbark tree (Quillaja saponaria), common corn-cockle (Agrostemma githago L.), baby's breath (Gypsophila spp.) and soybeans (Glycine max L.). They are used in soaps, medicines (e.g., drug adjuvants), fire extinguishers, dietary supplements, steroid synthesis, and in carbonated beverages (for example, being responsible for maintaining the head on root beer). Saponins are both water and fat soluble, which gives them their useful soap properties. Some examples of these chemicals are glycyrrhizin (licorice flavoring) and quillaia (alt. quillaja), a bark extract used in beverages.

Structurally, they are glycosides with at least one glycosidic linkage between a sugar chain (glycone) and another non-sugar organic molecule (aglycone).^{[citation needed]}

Steroid glycosides are saponins with 27-C atoms. They are modified triterpenoids where their aglycone is a steroid, these compounds typically consist of a steroid aglycone attached to one or more sugar molecules, which can have various biological activities. These compounds are known for their significant cytotoxic, neurotrophic and antibacterial properties. These may also be used for partial synthesis of sex hormones or steroids.

Triterpene glycosides are natural glycosides present in various plants, herbs and sea cucumbers and possess 30-C atoms. These compounds consist of a triterpene aglycone attached to one or more sugar molecules. Triterpene glycosides exhibit a wide range of biological activities and pharmacological properties, making them valuable in traditional medicine and modern drug discovery.

The saponins are a subclass of terpenoids, the largest class of plant extracts. The amphipathic nature of saponins gives them activity as surfactants with potential ability to interact with cell membrane components, such as cholesterol and phospholipids, possibly making saponins useful for development of cosmetics and drugs. Saponins have also been used as adjuvants in development of vaccines, such as QS-21, Matrix-M, and Quil A, an extract from the bark of Quillaja saponaria. This makes them of interest for possible use in subunit vaccines and vaccines directed against intracellular pathogens. In their use as adjuvants for manufacturing vaccines, toxicity associated with sterol complexation remains a concern.

Quillaja is toxic when consumed in large amounts, involving possible liver damage, gastric pain, diarrhea, or other adverse effects. The NOAEL of saponins is around 300 mg/kg in rodents, so a dose of 3 mg/kg should be safe with a safety factor (see Therapeutic index) of 100.

Saponins are used for their effects on ammonia emissions in animal feeding. In the United States, researchers are exploring the use of saponins derived from plants to control invasive worm species, including the jumping worm.

The principal historical use of these plants was boiling down to make soap. Saponaria officinalis is most suited for this procedure, but other related species also work. The greatest concentration of saponin occurs during flowering, with the most saponin found in the woody stems and roots, but the leaves also contain some.