Duncan Island, in the Susquehanna River at Martic Township in Lancaster County, Pennsylvania, is a prehistoric archaeological site that preserves evidence of human activity across a vast span of time, from the Archaic period through about 1000 A.D. Known as sites 36LA60 and 36LA61, it was investigated in the 1950s and again in the 1970s, when archaeologists recovered artifacts dating across that long period of prehistory. The site's significance as a record of ancient life in the region was formally recognized when it was added to the National Register of Historic Places in 1984.