Delaware Route 261 (DE 261), also known as Foulk Road, is a 4.62-mile (7.44 km) state highway north of Wilmington in New Castle County, Delaware. It runs from an interchange with U.S. Route 202 (US 202) and DE 141 north of Interstate 95 (I-95) near Fairfax northeast to the Pennsylvania state line, where it continues into that state as Pennsylvania Route 261 (PA 261). The road runs through suburban areas of Brandywine Hundred as a four-lane road south of DE 92 and a two-lane road north of DE 92.

DE 261 was originally designated in the 1930s. In the 1960s, most of the route was widened into a four-lane road. The southern terminus at US 202 was reconstructed into an interchange in the 2000s.

DE 261 begins at an interchange with US 202 and the northern terminus of DE 141 near Fairfax, heading northeast on four-lane divided Foulk Road. The road heads near business parks, becoming an undivided road. The route continues through the suburban Brandywine Hundred area, passing residences along with a few businesses. DE 261 remains a four-lane undivided road through this area, occasionally widening into a divided highway at intersections. The route intersects several roads including Murphy Road, Shipley Road, and Silverside Road. Farther northeast, DE 261 comes to the intersection with DE 92, at which point it turns north and becomes a two-lane undivided road that passes more homes. The route ends at the Pennsylvania state line, where Foulk Road continues into that state as PA 261.

DE 261 has an annual average daily traffic count ranging from a high of 22,555 vehicles at the Silverside Road intersection to a low of 10,414 vehicles at the Pennsylvania border. None of DE 261 is part of the National Highway System.

By 1920, what is now DE 261 existed as a county road. When Delaware first assigned state highway numbers by 1936, what is now DE 261 had been upgraded to a state highway, but did not receive a number at that time. By 1938, DE 261 was designated onto its current alignment between US 202 and the Pennsylvania border, continuing the PA 261 designation. DE 261 was widened into a four-lane road between US 202 and Silverside Road by 1966. The four-lane portion was extended north to Naamans Road a year later. In summer 2007, construction on converting the intersection with US 202 into interchange was completed as part of the Blue Ball Properties project, a project undertaken to improve roads in this area as part of AstraZeneca locating their North American headquarters to the area. DE 141 was also realigned to intersect US 202 and DE 261 at this interchange. The total cost of the project was $123 million.

The entire route is in New Castle County.