Physically strong yet technically adept, Sieloff soon got the attention of West Germany national team coach Sepp Herberger, who first called him up for a 1964 friendly against Finland in Helsinki.

Unfortunately for Sieloff, he lost his starting place in the West Germany national team during the 1965–66 season and thus did not feature in the 1966 FIFA World Cup.

During the next four years, the sweeper position was usually that of either Willi Schulz or Karl-Heinz Schnellinger and thus Sieloff did not gain anymore caps up to 1970.

With Sieloff being one of the celebrated stars of the team, he made his comeback for West Germany in April 1970 in a friendly against Romania.

However, during 1971 it became apparent that Franz Beckenbauer, who previously played in midfield, was the strongest option at the sweeper position, which meant the end of Sieloff's international career after 14 appearances.