Oldsum (Fering: Olersem) is a municipality on the island of Föhr, in the district of Nordfriesland, in Schleswig-Holstein, Germany.

Oldsum's landmark is an ancient thatched windmill whose antecessor presumably dates back to the year 1700.

Oldsum is situated approximately two kilometres from the western shore of the island, the northern coastline is a little closer.

Oldsum adjoins Süderende to the south, Dunsum to the southwest and Alkersum and Midlum to the east.

Oldsum underwent a transformation from a farmers' to an artists' village, numerous studios and galleries can now be found there.