Koarnjum (Dutch: Cornjum[3]) is a village in the municipality of Leeuwarden (Friesland province), in the Netherlands.

[4] The village was first mentioned in 1423 as Coernem, and means "settlement of the people of Curra".

[6] Koarnjum was served by a station on the North Friesland Railway which opened in 1901 and closed to passengers in 1930.

[6] The old mansion Martenastate in Koarnjum was demolished in 1899 and replaced by a small fantasy castle.

The last owner was Duco Martena van Burmania Vegelin van Claerbergen who was in financial difficulties and sold the ruinous castle to the local church in 1899 who demolished the castle and sold the land to a real estate agent.