Karl-Rudolf Silberg-Sillak (20 March 1906 – 17 December 1974) was an Estonian footballer, who made 52 appearances for the Estonia national team between 1929 and 1938.
He also made 12 appearances for the Estonia national bandy team and Estonia national ice hockey team.
Sillak was born in Märjamaa in 1906, and began playing for Sport in 1923.
He earned his first cap for the Estonian national team in 1929, and became a mainstay in the defensive line in the 1930s.