Eidum or Eydum was a historic place on the German island of Sylt in the North Sea. It was several hundred metres west of the present coast line of the present-day village of Westerland.
According to historic tradition the settlement of Eidum was destroyed several times by storm tides and then rebuilt again. The All Saints Day Flood of 1436 probably wreaked so much damage and so many casualties in the village of Eidum that survivors left it and founded a new settlement about 2 kilometres further east. Their choice alighted on the higher geest ridge in the area of Tinnum. This settlement was called Südhedig; from it arose the present day village of Westerland. The later name "Westerland" is supposed to have derived from an old Tinnum field name, on which this new settlement stood. The land was west of the village, hence "Wester-land".
The church tower of the old Eidum village survived the storm tide and was said to have been visible for a long time afterwards. It finally collapsed, according to contemporary sources, "at night and in calm weather". In any case the Westerland folk presented a petition for a grant towards the construction of a new village church, St. Neil's, two months before the Burchardi Flood im August 1634. The Eidum church was mentioned for the last time in the Dankwardt Chronicle of 1652.