Johann Reinhold Forster (German: [ˈfɔʁstɐ]; 22 October 1729 – 9 December 1798) was a German Reformed pastor and naturalist. Born in Dirschau, Pomeranian Voivodeship, Polish–Lithuanian Commonwealth (now Tczew, Poland), he attended school in Dirschau and Marienwerder before being admitted at the Joachimsthal Gymnasium in Berlin in 1745. Skilled in classical and biblical languages, he studied theology at the University of Halle. In 1753, he became a parson at a parish just south of Danzig. He married his cousin Justina Elisabeth Nicolai in 1754, and they had seven children; the oldest child was George Forster, also known as Georg.

In 1765, Forster was commissioned by the Russian government to inspect the new colonies on the Volga. Accompanied by George on the journey, he observed the conditions of the colonists and made scientific observations that were later read at the Russian Academy of Sciences. After making a report that was critical of the Russian administration, Forster left for England without payment in 1766. In England, Forster became the successor of Joseph Priestley as tutor in modern languages and natural history at Warrington Academy where he worked for two years. He made contact with many other naturalists, published a textbook on mineralogy and translated works of the apostles of Linnaeus into English. Invited by the geographer Alexander Dalrymple, Forster moved to London in 1770 in preparation for participation in an East India Company expedition, but the plans fell through. Forster continued to publish translations and scientific works including contributions to North American zoology and botany. In February 1772, he was elected a Fellow of the Royal Society.

After the withdrawal of Joseph Banks from the second voyage of James Cook, Forster accepted the position of naturalist on Cook's ship, where he was accompanied by his son George as draughtsman and assistant. On their three-year journey, they made the first recorded crossing of the Antarctic Circle and made observations and discoveries in New Zealand and Polynesia. When they returned to England, Forster published the botanical work Characteres generum plantarum. However, there was disagreement with Cook on who should write a narrative of the journey. After a lengthy argument, George wrote A Voyage Round the World, which appeared six weeks before Cook's account. Forster separately published his scientific Observations Made During a Voyage Round the World.

Forster's pride and obstinacy caused him to fall out with many powerful men in England; after clearing some of his substantial debt with the aid of German patrons, he returned to Germany where he was a professor at the University of Halle from 1780. He oversaw the university's botanical garden for a few years and published in a wide range of sciences. Forster died in 1798 and is buried in Halle. He is commemorated in the names of various species of plants and animals, including the genera Forstera and Forsterygion.

Johann Reinhold Forster was born on 22 October 1729 in Dirschau, Pomeranian Voivodeship, Polish–Lithuanian Commonwealth (now Tczew, Poland). Forster's family traced its origins to a Yorkshire-born ancestor called George Forster, a descendant of the Scottish Forrester family. This ancestor had emigrated from England after supporting the losing Royalist side in the English Civil War and had become a merchant in Neuenburg (Nowe) in the 1640s. Forster's father was Georg Reinhold Forster and his mother was Eva Plaht, the daughter of Dirschau's mayor Johann Wolff and the widow of Martin Plaht. Georg Reinhold became mayor of Dirschau in 1733 (as the third Forster in this position) but became an invalid in 1735 and had to retire as mayor. Forster attended a Latin school in Dirschau, and then went to Marienwerder (Kwidzyn) to attend the gymnasium in 1743.

In May 1745, Forster was admitted at the Joachimsthalsches Gymnasium in Berlin, a reputable institution. His teachers included Johann Philipp Heinius [de], the school rector, who taught theology, Hebrew, Roman antiquity and philosophy; and Friedrich Muzel [de], the school librarian, who taught history, literature, Latin and natural philosophy. Forster quickly became skilled in classical and biblical languages including Coptic and fluent in Latin. He graduated in 1748 together with Karl Franz von Irwing [de], who became a lifelong friend. Other friends and acquaintances included Louis de Beausobre, August Friedrich Pallas [de] and Carl Gottfried Woide. Forster also sought out the friendship of foreign students to improve his knowledge of modern languages, especially French.

After his time at the gymnasium, Forster wanted to study medicine at the University of Halle but his father wanted him to study law. As a compromise, Forster studied theology in Halle. Little is known about his student days. His academic teachers included theologian Siegmund Jakob Baumgarten and orientalist Christian Benedikt Michaelis [de].

By July 1751, Forster had arrived in Danzig (Gdańsk), where he was a candidate for ordination at the Reformed church of St Peter and Paul [de; pl]. He had a meagre income, and his preaching workload did not allow him to pursue his scholarly interests. His sermons were praised, and his superiors contemplated sending him abroad for additional training. Forster was ordained in Königsberg in August 1753 and (possibly to please his father) accepted the post of parson of the Hochzeit-Nassenhuben parish just south of Danzig, starting there on 23 September 1753.