Johann Wilhelm Hittorf (27 March 1824 – 28 November 1914) was a German physicist who was born in Bonn and died in Münster, Germany.

Hittorf was the first to compute the electricity-carrying capacity of charged atoms and molecules (ions), an important factor for understanding electrochemical reactions.

He experimented with tubes containing energy rays extending from a negative electrode.

Between 1853 and 1859 his most important work concerned ion movement caused by electric current.

This observation resulted in the concept of transport number, the fraction of the electric current carried by each ionic species.