Thorvald (Thorwald) Julius Sørensen (4 July 1902 – 21 June 1973) was a Danish botanist and evolutionary biologist.

[1] Thorvald Sørensen spent the years 1931–1935, based on Ella Island, studying plants in the then little known North-East Greenland.

[2] He also 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 illustrated its use on a data set collected in Danish grassland by Johannes Grøntved (1882-1956), who used the quantitative method to sample vegetation developed by Christen C. Raunkiær (1860–1938).