Thora Gerda Sofie Castenschiold (1 February 1882 – 30 January 1979) was a Danish tennis player at the beginning of the 20th century. She was the first woman to represent Denmark at the Olympics.
Castenschiold, a member of Copenhagen's KB, won the title at the first Danish women's tennis championships in 1906. She could defend it four times in a row until 1910.
She took part in the 1912 Summer Olympics at Stockholm and won the silver medal in the indoor singles competition.