Darby is a borough in Delaware County, Pennsylvania, United States, along Darby Creek southwest of downtown Philadelphia. Darby was settled about 1654 and inhabited by Quakers early in the colonial era. Incorporated on March 3, 1852, it had 3,429 residents in 1900, 6,305 in 1910, 10,334 in 1940, and 10,687 at the 2010 census.
Comedian and actor W.C. Fields was born in 1880 at the Arlington Hotel, then located at 832 Main Street.
Darby is home to the fifth-oldest all-volunteer Fire Department and second-oldest free library in the United States, founded in 1743. One of its cemeteries is more than 300 years old.
Darby Borough is distinct from the nearby municipality of Darby Township.