Giardia duodenalis, also known as Giardia intestinalis and Giardia lamblia, is a flagellated parasitic microorganism of the genus Giardia that colonizes the small intestine, causing a diarrheal condition known as giardiasis. The parasite attaches to the epithelium by a ventral adhesive disc or sucker, and reproduces via binary fission. Giardiasis does not spread via the bloodstream, nor does it spread to other parts of the gastrointestinal tract, but remains confined to the lumen of the small intestine. Giardia has an outer membrane that makes it possible to retain life, even when outside of the host body, and which can make it tolerant to chlorine disinfection. Giardia trophozoites absorb their nutrients from the lumen, and are anaerobes. If the organism is split and stained, its characteristic pattern resembles the familiar "smiley face" symbol.

Chief pathways of human infection include ingestion of untreated drinking water (which is the most common method of transmission for this parasite), food, and soil contaminated with human feces, as well as ingestion of sewage, a phenomenon particularly common in many developing countries. Contamination of natural waters also occurs in watersheds where intensive grazing occurs.

Giardia infections occur worldwide. It is the most commonly identified intestinal parasite in the United States and Canada among children in day-care centers, hikers, family members, and immunocompromised adults. About 20,000 cases per year in the United States are reported.