Joseph Edward Duncan III (February 25, 1963 – March 28, 2021) was an American convicted serial killer and child molester who was on death row in federal prison following the 2005 kidnappings and murders of members of the Groene family of Coeur d'Alene, Idaho. He was also serving 11 consecutive sentences of life without parole for the 1997 murder of Anthony Martinez of Beaumont, California. Additionally, Duncan confessed to — but had not been charged with — the 1996 murder of two girls, Sammiejo White and Carmen Cubias, in Seattle, Washington. At the time of the attack on the Groene family, Duncan was on the run from a child molestation charge in Minnesota.

During his incarceration, authorities connected Duncan with the unsolved murders of Anthony Martinez in California and two girls in Seattle, which all occurred when Duncan was on parole from 1994 to 1997. In all, Duncan was convicted in Idaho for kidnapping and murdering the three victims in Coeur d'Alene, for which he was given six life sentences: in federal court for kidnapping Shasta and Dylan Groene and murdering Dylan, for which he was given three death sentences and three life sentences; and in the state of California for kidnapping and murdering Martinez, for which he was given two life sentences.

Duncan died on March 28, 2021, at the age of 58, as a result of a terminal brain tumor.