Dietmar Heinrich Nietan (born 25 May 1964) is a German politician of the Social Democratic Party (SPD) who has been serving as a member of the Bundestag for the state of North Rhine-Westphalia from 1998 to 2002 and again since 2005. In addition to his work in parliament, he has been the Coordinator of German-Polish Intersocietal and Cross-Border Cooperation at the Federal Foreign Office since 2022, under the coalition government of Chancellor Olaf Scholz. In 2014, Nietan became the SPD's treasurer, a role that positioned him within the party's national leadership under the current co-chairs, Saskia Esken and Lars Klingbeil.