Swedish physician and botanist born in Erikstad in the Vittaryd parish in Småland, who trained in Lund (1746) and Uppsala (1749). He was a student of Linnaeus and defended his disseration in 1750. Bergius had a medical practice in Stockholm (1754-1761) and was later appointed professor of natural history and pharmacy (1761) at the Collegium Medicum in Stockholm.

Though he described South African plants in his book Descriptiones Plantarum ex Capita Bonae Spei (1767), and types of South African plants are based on Bergius specimens, the material came from other collectors and particularly Mikael Grubb, director and banker for the Swedish East India Company. Much of this material from Grubb was purchased from the original collector, J.A. Auge, during a return journey of Grubb from China (1764). Bergius would also have had access to South African material collected by P.J. Bladh, C.G. Ekeberg, J.G. König, P. Osbeck, P. Sonnerat, A. Sparrman and C.P. Thunberg. Other African material in his herbarium included North African specimens from Egypt, collected by F. Hasselquist, and from Libya collected by G. Rothman. Specimens from Mauritius and Réunion were originally collected by P. Sonnerat and subsequently cultivated by A. Thouin. Specimens from the Comoros were collected by J.G. König and P. Osbeck, while material from Madagascar was collected by P. Osbeck and P. Sonnerat.

P.J. Bergius is commemorated by the genus Bergia L. in the Elatinaceae. He bequeathed his estate (the basis for the Bergius Foundation), including his library, herbarium and the private botanical garden originally created with his brother Bengt, to the Royal Swedish Academy of Sciences in Stockholm. The Bergius Botanic Garden (Bergianska Trädgården) was originally located with their mansion in the Vasastaden district of central Stockholm and donated for public benefit and to provide horticultural training (1791). It was relocated in 1885 to the Frescati area, close to the Swedish Museum of Natural History, and is now administered jointly by the Royal Swedish Academy of Sciences and Stockholm University. The Bergius Herbarium contains material sent to P.J. Bergius by contemporary collectors and was added to after his death by O. Swartz and J.E. Wikström, therefore including later material up to c. 1830. The historical herbarium contains over 17,000 sheets and is located at the Bergius Botanic Garden.