Detlef Seif (born 15 August 1962 in Euskirchen) is a German politician of the Christian Democratic Union (CDU) who has been a Member of the German Bundestag since 2009.

On a scholarship of the Konrad Adenauer Foundation, Seif studied law at the University of Bonn from 1983 until 1989. During his studies, he worked for the International Society for Human Rights. Since 1993, he has been practicing as a lawyer.

Seif joined the CDU in 1982 and held positions in local politics in Weilerswist from 1999 to 2009.

Since 2009, Seif has been CDU party chairman in the district of Euskirchen.

Seif has been a member of the German Bundestag since the 2009 federal elections. Since then, he has been a member of the Committee on European Affairs. In 2014 he became Deputy Spokesman for the committee. Seif's thematic areas of expertise in the committee are basic rights; EU citizenship; coordination between the judicature and police; border protection and counterterrorism; and asylum- and migration policy. Additionally, he is the CDU/CSU parliamentary group's rapporteur for Great Britain and Malta.

Between 2009 and 2017, Seif was a member of the Committee on Legal Affairs, where he served as his parliamentary group's rapporteur on legal professionals, data privacy and labor law.

In 2018, he joined the Committee on Internal Affairs.

The same year, Seif became a member of the first Investigative Committee of the 19th legislative period, which is investigating the failure of German security authorities to prevent Anis Amri's attack on a Berlin Christmas market in December 2016.