Friedrich Oskar Giesel (20 May 1852 – 13 November 1927, known as Fritz) was a German organic chemist.
During his work in a quinine factory in the late 1890s he started to work on the at-that-time-new field of radiochemistry and started the production of radium.
In the period between 1902 and 1904 he was able to isolate a new element emanium.
In a now controversially reviewed process it was stated that emanium is identical to actinium, which was discovered by André-Louis Debierne in 1899.