Research on informal housing tends to focus overwhelmingly on less developed countries, downplaying or ignoring entirely the presence of informality in United States housing markets. In actuality, a longstanding and widespread tradition of informal housing exists in the United States but is typically disregarded by scholars. In this article we draw on three definitions of informality—as non-compliant, non-enforced, or deregulated economic activity—to characterize examples of informality in US housing markets, focusing in particular on five institutions that govern housing market activity in this country: property rights law, property transfer law, land-use and zoning, subdivision regulations, and building codes. The cases presented here challenge the notion that informality is absent from US housing markets and highlight the unique nature of informal housing, US style—namely, that informal housing in the US is geographically uneven, largely hidden and typically interwoven within formal markets. We conclude with a discussion of how research on informal housing in the US can inform research in the global South.