Johann Reinhold Forster (October 22, 1729 - December 9, 1798) was a Polish born naturalist of German descent.

Forster made contributions to the early ornithology of Europe and North America. He is best known as the naturalist on James Cook's second Pacific voyage, when he was accompanied by his son Georg Forster.

Forster was born in the Polish povince called Royal Prussia in the city of Tczew, twenty miles south of Gdansk. He studied theology at the University of Halle, Germany, afterwards serving as a Lutheran pastor. In 1766 he travelled to England with his son Georg. He spent three years teaching in Warrington before moving to London, where he became known as a natural historian. When Joseph Banks withdrew at the last moment as naturalist on Cook's second voyage, Forster and his son were appointed to fill the vacant position. In July 1772 they set sail on the Resolution, returning to England in July 1775. During a stop in Cape Town, Forster engaged Anders Sparrman to act as his assistant.

Both the Forsters kept detailed diaries of everything they saw on the voyage, and made extensive collections of both natural history specimens and artefacts. On his return Forster published Observations Made during a Voyage round the World (1778). In November 1779 he was appointed Professor of Natural History and Mineralogy at the University of Halle, where he remained until his death.