Johann Heinrich Gelzer (17 October 1813 – 15 August 1889) was a Swiss historian and diplomat who was a native of Schaffhausen.

From 1844 to 1851, he held the chair of history at Humboldt University in Berlin, and in the meantime, served as an adviser to Frederick William IV of Prussia.

In 1851 he returned to Basel, where he edited the Protestantischen Monatsblätter für innere Zeitgeschichte.

In 1856–1857 he worked as a mediator between Prussia and Switzerland in regards to the Neuenburger Handel (Neuchâtel affair).

One of his better written efforts was the two-volume Die letzten drei der Jahrhunderte Schweizergeschichte (The Last Three Centuries of Swiss History).