Sigurður Gunnar Þorsteinsson (born 8 July 1988) is an Icelandic professional basketball player who plays for Úrvalsdeild karla club Tindastóll and the Icelandic national basketball team. Nicknamed Ísafjarðartröllið (English: The Ísafjördur Troll), he has won the Icelandic championship three times, in 2008, 2012 and 2013. Outside of Iceland, Sigurður has played professionally in Greece and Sweden.