Stefan Birkner (born 12 April 1973) is a German lawyer and politician for the Free Democratic Party (FDP). From 2011 to 2023, he chaired the FDP in Lower Saxony.

Birkner was elected to the State Parliament of Lower Saxony in the 2008 state elections. However, in February 2008 he left the Landtag to become a State Secretary for the State Minister of the Environment in the second cabinet Wulff, Hans-Heinrich Sander. He was replaced by Christian Grascha. From January 2012 until 2013, he served as State Minister for Environment, Energy and Climate Protection in the government of Minister-President David McAllister.

In September 2017, Birkner succeeded Christian Dürr as chairman of the FDP's parliamentary group.

In the negotiations to form a so-called traffic light coalition of the Social Democrats (SPD), the Green Party and the FDP following the 2021 federal elections, Birkner led his party's delegation in the working group on environment policy; his co-chairs from the other parties were Rita Schwarzelühr-Sutter (SPD) and Steffi Lemke (Bündnis 90/Die Grünen).

Birkner was nominated by his party as a delegate to the Federal Convention to elect the President of Germany in 2022.

In February 2023, Birkner joined consulting firm Ernst & Young. In April 2023, he was nominated by Federal Minister for Digital and Transport Volker Wissing to lead the executive board of the Autobahn GmbH, a state-owned entity; amid public criticism around the appointment, he withdrew his name from consideration.

Birkner was born in Münsterlingen.

He is married. His brother-in-law is fellow politician Robert Habeck.