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.

Like all unprotected islands, Hochsände, and sandbanks off the west coast of Schleswig-Holstein, Tertius is migrating steadily eastwards.

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

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.