Ole Theodor Jensen Mortensen, also known as Theodor Mortensen was a Danish scientist and professor at the Zoological Museum, Copenhagen. He specialized in sea urchins (Echinoidea) and provided an enormous marine collection to the museum. He collected many sea urchin species on his expeditions between 1899–1930. Between the years of 1914 to 1916 Mortensen undertook a research expedition to the Pacific.