Tertius (also known as Tertiussand) is a sandbank located in the Heligoland Bight off the coast of Dithmarschen belonging to the German state of Schleswig-Holstein.

Formerly a Hochsand (a sandbank rising above the ordinary high water mark, effectively an island), Tertius has been reduced in elevation by coastal erosion in recent decades.

According to the Schleswig-Holstein National Park Office in Tönning, Tertius, like Blauort, belongs to the parish of Hedwigenkoog in the district of Dithmarschen.

However, the state government of Schleswig-Holstein has not yet officially confirmed and authorised this, so the sandbank technically remains an unorganised location.

Though the sandbank is not part of the highly protected Zone 1 of the Schleswig-Holstein Wadden Sea National Park, public access to Tertius is subject to restrictions.