WHOIS (pronounced as the phrase "who is") is a query and response protocol that is used for querying databases that store an Internet resource's registered users or assignees. These resources include domain names, IP address blocks and autonomous systems, but it is also used for a wider range of other information. The protocol stores and delivers database content in a human-readable format. The current iteration of the WHOIS protocol was drafted by the Internet Society, and is documented in RFC 3912.

Whois is also the name of the command-line utility on most UNIX systems used to make WHOIS protocol queries. In addition, WHOIS has a sister protocol called Referral Whois (RWhois).

Elizabeth Feinler and her team (who had created the Resource Directory for ARPANET) were responsible for creating the first WHOIS directory in the early 1970s. Feinler set up a server in Stanford's Network Information Center (NIC) which acted as a directory that could retrieve relevant information about people or entities. She and the team created domains, with Feinler's suggestion that domains be divided into categories based on the physical address of the computer.

The process of registration was established in RFC 920. WHOIS was standardized in the early 1980s to look up domains, people, and other resources related to domain and number registrations. As all registration was done by one organization at that time, one centralized server was used for WHOIS queries. This made looking up such information very easy.

At the time of the emergence of the internet from the ARPANET, the only organization that handled all domain registrations was the Defense Advanced Research Projects Agency (DARPA) of the United States government (created during 1958.). The responsibility of domain registration remained with DARPA as the ARPANET became the Internet during the 1980s. UUNET began offering domain registration service; however, they simply handled the paperwork which they forwarded to the DARPA Network Information Center (NIC). Then the National Science Foundation directed that commercial, third-party entities would handle the management of Internet domain registration. InterNIC was formed in 1993 under contract with the NSF, consisting of Network Solutions, Inc., General Atomics and AT&T. The General Atomics contract was canceled after several years due to performance issues.

20th-century WHOIS servers were highly permissive and would allow wild-card searches. A WHOIS query of a person's last name would yield all individuals with that name. A query with a given keyword returned all registered domains containing that keyword. A query for a given administrative contact returned all domains the administrator was associated with. Since the advent of the commercialized Internet, multiple registrars and unethical spammers, such permissive searching is no longer available.

On December 1, 1999, management of the top-level domains (TLDs) com, net, and org was assigned to ICANN. At the time, these TLDs were converted to a thin WHOIS model. Existing WHOIS clients stopped working at that time. A month later, it had self-detecting Common Gateway Interface support so that the same program could operate a web-based WHOIS lookup, and an external TLD table to support multiple WHOIS servers based on the TLD of the request. This eventually became the model of the modern WHOIS client.

By 2005, there were many more generic top-level domains than there had been in the early 1980s. There are also many more country-code top-level domains. This has led to a complex network of domain name registrars and registrar associations, especially as the management of Internet infrastructure has become more internationalized. As such, performing a WHOIS query on a domain requires knowing the correct, authoritative WHOIS server to use. Tools to do WHOIS domain searches have become common and are offered by providers such as IONOS and Namecheap.