Gustav Friedrich Carl Johann Sthamer (24 November 1856 – 29 June 1931) was a German lawyer, First Mayor of Hamburg, and ambassador.

Sthamer was born on 24 November 1856 in Groß Weeden, part of the municipality Rondeshagen.

He studied law at the universities of Heidelberg, Leipzig, and Göttingen.

During the First World War, Sthamer was president of the civil government of the occupied Antwerp.

After a short term in office, Sthamer resigned because he was appointed as German ambassador to the United Kingdom.