Thiomargarita namibiensis is a Gram-negative coccoid bacterium, found in the ocean sediments of the continental shelf of Namibia. It is the second largest bacterium ever discovered, as a rule 0.1–0.3 mm (100–300 μm) in diameter, but sometimes attaining 0.75 mm (750 μm). Cells of Thiomargarita namibiensis are large enough to be visible to the naked eye. Although the species held the record for the largest known bacterium, Epulopiscium fishelsoni – previously discovered in the gut of surgeonfish – grows slightly longer, but narrower.

Thiomargarita means "sulfur pearl". This refers to the appearance of the cells; they contain microscopic sulfur granules that scatter incident light, lending the cell a pearly lustre. Like many coccoid bacteria such as Streptococcus, their cellular division tends to occur along a single axis, causing their cells to form chains, rather like strings of pearls. The species name namibiensis means "of Namibia".