A giant cell (also known as a multinucleated giant cell, or multinucleate giant cell) is a mass formed by the union of several distinct cells (usually histiocytes), often forming a granuloma. Although there is typically a focus on the pathological aspects of multinucleate giant cells (MGCs), they also play many important physiological roles. Osteoclasts are a type of MGC that are critical for the maintenance, repair and remodeling of bone and are present normally in a healthy human body. Osteoclasts are frequently classified and discussed separately from other MGCs which are more closely linked with disease.

Non-osteoclast MGCs can arise in response to an infection, such as from tuberculosis, herpes, or HIV, or a foreign body. These MGCs are cells of monocyte or macrophage lineage fused together. Similar to their monocyte precursors, they are able to phagocytose foreign materials. However, their large size and extensive membrane ruffling make them better equipped to clear up larger particles. They utilize activated CR3s to ingest complement-opsonized targets. Non-osteoclast MGCs are also responsible for the clearance of cell debris which is necessary for tissue remodeling after injuries.

Types include foreign-body giant cell, Langhans giant cell, Touton giant cells, Giant-cell arteritis, and Reed–Sternberg cell.