Abstract: The pharaoh ant, Monomorium pharaonis (Linnaeus, 1758), has long been considered the most ubiquitous house ant in the world. Monomorium pharaonis is particularly notorious as a pest in hospitals, where it is known as a vector for disease. I compiled and mapped specimen records of M. pharaonis from > 1200 sites to document its known worldwide distribution and evaluate hypotheses concerning its geographic origin. I documented the earliest known M. pharaonis records for 225 geographic areas (countries, island groups, major islands, Us states, Canadian provinces, and Russian federal districts), including many for which I found no previously published records: Austral Islands, Central African Republic, Delaware, Dominican Republic, Gabon, Guatemala, Hainan Island, Haiti, Iraq, Ivory Coast, Maine, Missouri, Montana, Nebraska, New Hampshire, Pakistan, Palmyra Atoll, Rhode Island, Tunisia, and West Virginia. In tropical areas, M. pharaonis occurs both indoors and out, but in temperate areas, it is found almost exclusively indoors. It is by far the most common tropical ant found in heated buildings of Europe and North America. Monomorium pharaonis appears to have originated in tropical Asia, where widespread outdoor records have been reported. Also, Monomorium longi Forel, 1902 and Monomorium wroughtoni Forel, 1902, the two species thought to be most closely related to M. pharaonis, are endemic to tropical Asia. Although M. pharaonis was first described from Egypt, I found no evidence supporting the popular, but apparently mistaken, idea that M. pharaonis is native to Africa. Numerous authors contend that M. pharaonis populations are rapidly expanding. My analyses, however, suggest that M. pharaonis had already spread over much of the world > 100 years ago. Much of the purportedly recent population increases of M. pharaonis may be an artifact of greater sampling and dissemination of information.