Summer monsoonal rains (the southwest monsoon) are an important source of moisture for parts of the southwestern United States and northern Mexico. Improved documentation of the variability in the southwest monsoon is needed because changes in the amount and seasonal distribution of precipitation in this semiarid region of North America influence overall water supply and fire severity. Comparison of abundance variations in the planktic foraminifer Globigerinoides sacculifer in marine cores from the western and northern Gulf of Mexico with terrestrial proxy records of precipitation (tree-ring width and packrat-midden occurrences) from the southwestern United States indicate that G. sacculifer abundance is a proxy for the southwest monsoon on millennial and submillennial time scales. The marine record confirms the presence of a severe multicentury drought centered ca. 1600 calendar (cal.) yr B.P. as well as several multidecadal droughts that have been identified in a long tree-ring record spanning the past 2000 cal. yr from west-central New Mexico. The marine record further suggests that monsoon circulation, and thus summer rainfall, was enhanced in the middle Holocene (ca. 6500–4500 14C yr B.P.; ca. 6980–4710 cal. yr B.P.). The marine proxy provides the potential for constructing a highly resolved, well-dated, and continuous history of the southwest monsoon for the entire Holocene.