Arnold Friedrich Georg Diestel (2 March 1857 – 3 January 1924) was a German politician and a First Mayor of Hamburg.

In this function Diestel continued during the Weimar Republic, until the complete senate resigned on 27 March 1919, thus ending the life-term mandates under Hamburg's old 1860 constitution.

Diestel was associated, but not a member of the German Democratic Party (Deutsche Demokratische Partei (DDP)).

DDP and SPD had formed a coalition majority in the Hamburg Parliament.

On 2 February 1920 the fellow senators elected Diestel their president and First Mayor of Hamburg, taking office after the announced resignation of Friedrich Sthamer, becoming ambassador.