Henry Friedrich Wilhelm Siedentopf (22 September 1872 in Bremen – 8 May 1940 in Jena) was a German physicist and pioneer of microscopy.
In 1902 the ultramicroscope was developed by Richard Adolf Zsigmondy (1865–1929) and Siedentopf, working for Carl Zeiss AG.
In 1925, Zsigmondy received the Nobel Prize for Chemistry also for this work.
In 1908, together with August Köhler, he invented the fluorescence microscope.
In 1930 he was elected a member of the German National Academy of Sciences Leopoldina.