Dionísio Silva Castro (born 22 November 1963 in Fermentões-Guimarães) is a former long-distance runner from Portugal, best known for setting the world record in the 20,000 metres on 31 March 1990, when he clocked 57:18.4 in La Flèche. Castro continued for another 943 metres in an attempt to break Jos Hermens' world record for the one-hour run as well, but missed out by one metre. He competed for the Portuguese club Sporting Clube de Portugal. His twin brother Domingos was also a world class athlete in the long distance events. The two of them represented their native country at the 1988 (Seoul, South Korea) and 1992 Summer Olympics (Barcelona, Spain).

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