J. De Deken was a Belgian international foil fencer from Antwerp in the early 20th-century. She was trained by the Belgian fencing master Verbrugge. She was a short fencer, and her style has been described as "graceful".
In 1914 De Deken was one of the two Belgian women who competed at the 1914 Salle Giandomenici international women's fencing competition in Amsterdam. This competition was the first women's fencing competition in the Netherlands with Dutch, British and Belgian competitors. She had a very nice match during the competition. Overall she was the best Belgian women and won the fourth prize behind C. E. Martin Edmunds, Esther Bekkers-Lopes Cardozo and Rosamund Johnstone.
In May 1914 she won the third prize at the international 1914 "La Femme Contemporaine" women's fencing competition in Antwerp.