Thorvald (Thorwald) Julius Sørensen (1902–1973) was a Danish botanist and evolutionary biologist.
Sørensen was professor at the Royal Veterinary and Agricultural College 1953–1955 and at the University of Copenhagen 1955-1972. He was director of the Copenhagen Botanical Garden and Botanical Museum during the same period.
Thorvald Sørensen spent the years 1931–1935, based on Ella Island, studying plants in the then little known North-East Greenland. He published a doctoral thesis on the annual phenological rhythm of the High Arctic plant species, including the pollination of their flowers (1941).
He critically revised the Greenland flora and sorted out taxonomy of a number of difficult taxa, most notably Puccinellia.
He carried out a number of studies in the evolutionary biology of plants, such as Taraxacum, Capsella bursa-pastoris and Ranunculus.
He developed a quotient of similarity in species composition between plant communities—the still much-used Sørensen similarity index. He illustrated its use on a data set collected in Danish grassland by Johannes Grøntved, who used Raunkiær's quantitative method to sample vegetation.
Example of plant name he published:
- Braya intermedia T.J.Sørensen (1954) in Meddelelser om Grønland 134 (8).
- Puccinellia groenlandica T.J.Sørensen (1953) in Meddelelser om Grønland 134 (3).
The Arctic campion species Silene sorensensis was named to his honour.