Christiani & Nielsen was a construction contractor with major operations worldwide.

Christiani & Nielsen was established by Rudolf Christiani, a Danish civil engineer, and Aage Nielsen, a captain in the Royal Danish Navy, in Copenhagen, to build bridges, marine works, and other reinforced concrete structures, in 1904.

[1] It soon established a branch in Hamburg, and after the First World War, extended its operations to the United Kingdom, South America, Australia, and South Africa.

[2] The company's business in Thailand was demerged on the Stock Exchange of Thailand in 1991 and continued operating under different ownership.

[3] The main company went into administration in November 2000.