.us is the Internet country code top-level domain (ccTLD) for the United States of America, established in 1985. Registrants of .us domains must be United States citizens, residents, or organizations, or a foreign entity with a presence in the United States. Most registrants in the country have registered for .com, .net, .org and other gTLDs, rather than .us, which has traditionally primarily been used by many state and local governments (although any entity had the option of registering a .us domain.) In particular, the domains .gov and .mil have been reserved for US usage.

The original administrator of .us was Jon Postel of the Information Sciences Institute at the University of Southern California. He administrated .us under a sub-contract USC/ISI had from SRI International (who held the .us and the Generic top-level domains contract with the United States Department of Defense) and later Network Solutions (who held the .us and the Generic top-level domains contract with the National Science Foundation.) Registrants could only register third-level domains or higher in a geographic and organizational hierarchy. The vast majority of the geographic sub-domains in .us were delegated to various private entities and .us registrants would register with the delegated administrator for the level they wished to register in (not directly with the .us administrator.) However, from April 2002, second-level domains became available for registration. The .us domain is currently administered by NeuStar Inc. under a United States Department of Commerce contract.