Diederik Maarten Samsom; born 10 July 1971) is a Dutch environmentalist and retired politician who served the Labour Party (Partij van de Arbeid PvdA) from 2012 to 2016. He was the first leader in the 70 year history of the PvdA to have been voted out of his position by party members. Since November 2019, Samsom is head of cabinet for First Vice-President of the European Commission Frans Timmermans.
Elected to the House of Representatives between 30 January 2003 and 14 December 2016, he was elected PvdA parliamentary leader as well as party leader on 16 March 2012. He lost the Labour Party leadership election to Lodewijk Asscher in December 2016. Before his election to the House of Representatives he was CEO of a green energy company and a campaigner for Greenpeace Netherlands.