Least Nighthawk is widespread across eastern South America. This species appears to be most common in two disjunct regions: in southern Venezuela, Guyana and north central Brazil, and also in the cerrados of south central Brazil. The distribution also extends west, in the north, to eastern Colombia; there are smaller populations across central Brazil; and in the south, Least Nighthawk also occurs in eastern Bolivia and northeastern Argentina.
Many populations of Least Nighthawk are thought to be sedentary, although the movement patterns for the species as a whole are poorly known. Friedmann (1948) suggested that it might be migratory in the Río Negro region of Venezuela (subspecies esmeraldae); during one visit to the region Least Nighthawk was encountered "in any number" on only two days, leading Friedmann to wonder "if the species may be migratory, and whether the meeting with it was due to a sudden flight". Other explanations are possible, of course, including local movements, rather than long distance migrations. Dickerman (1988) suggested that migration might account for a record of the northern subspecies septentrionalis from Acre, west central Brazil. Sick (1997) described Least Nighthawk as migratory in central and southern Brazil, although without describing its movements in any detail. Kirwan et al. (2004) also noted that this species occurs in São Paulo and Minas Gerais, Brazil, "on a distinctly seasonal basis".