Oldsum (Fering: Olersem) is a municipality on the island of Föhr, in the district of Nordfriesland, in Schleswig-Holstein, Germany.
The municipality of Oldsum consists of the three hamlets of Oldsum, Klintum (Fering: Klantem) and Toftum (Fering: Taftem) which are spread over some two kilometres along a main road. The population number about 600. The landscape is marked by well-preserved thatched farmhouses. Oldsum's landmark is an ancient thatched windmill whose antecessor presumably dates back to the year 1700. Though burned down 200 years later, it was rebuilt and subsequently was in use until 1954. Since 1972 the mill is exclusively used as a dwelling house. 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 was first recorded in 1462 as Uluersum. During the 17th and 18th centuries, Oldsum, Klintum and Toftum were important whaling villages. A census in 1787 showed that the three places together had 961 inhabitants, 211 of whom were seafarers. One of the most successful whalers, Matthias Petersen (1632–1706) lived in Oldsum proper. In his lifetime he was able to catch 373 whales, his tomb can still be visited in the graveyard of the St. Laurentii church in Süderende.