Dimitrios Loundras (; 6 September 1885 – 15 February 1970) was a Greek gymnast and naval officer who competed at the 1896 Summer Olympics in Athens.
Loundras competed in the team parallel bars event. In that competition, Loundras was a member of the Ethnikos Gymnastikos Syllogos team that placed third of the three teams in the event. At 10 years 218 days, he remains the youngest medalist and competitor in Olympic history, if one discounts an unknown competitor of disputed age who competed as coxswain for the Dutch coxed pair rowing team in the 1900 Olympics.
Loundras later became an officer in the Royal Hellenic Navy, graduating from the Hellenic Navy Academy as an ensign in 1905. He served in various commands as well as a naval attache, and fought in World War I, before retiring with the rank of rear admiral in 1935. On the outbreak of the Greco-Italian War in 1940, he was recalled to active service and appointed head of the Aegean Naval Command. He finally retired in 1945 as a vice admiral.