A modular connector is a type of electrical connector for cords and cables of electronic devices and appliances, such as in computer networking, telecommunication equipment, and audio headsets.

Modular connectors were originally developed for use on specific Bell System telephone sets in the 1960s, and similar types found use for simple interconnection of customer-provided telephone subscriber premises equipment to the telephone network. The Federal Communications Commission (FCC) mandated in 1976 an interface registration system, in which they became known as registered jacks. The convenience of prior existence for designers and ease of use led to a proliferation of modular connectors for many other applications. Many applications that originally used bulkier, more expensive connectors have converted to modular connectors. Probably the best-known applications of modular connectors are for telephone and Ethernet.

Accordingly, various electronic interface specifications exist for applications using modular connectors, which prescribe physical characteristics and assign electrical signals to their contacts.

Modular connectors are often referred to as modular phone jack and plug, RJ connector, and Western jack and plug. The term modular connector arose from its original use in modular wiring components of telephone equipment by the Western Electric Company in the 1960s. This includes the 6P2C used for telephone line connections and 4P4C used for handset connectors.

Registered jack designations describe the signals and wiring used for voice and data communication at customer-facing interfaces of the public switched telephone network (PSTN). It is common to use a registered jack number to refer to the physical connector itself; for instance, the regular 8P8C modular connector type is often labeled RJ45 because the registered jack standard of the similar name RJ45S specified a similar, but modified, 8P8C modular connector. Similarly, various six-position modular connectors may be called RJ11. Likewise, the 4P4C connector is sometimes called RJ9 or RJ22 though no such official designations exist.^{[citation needed]}

The first types of small modular telephone connectors were created by AT&T in the mid-1960s for the plug-in handset and line cords of the Trimline telephone. Driven by demand for multiple sets in residences with various lengths of cords, the Bell System introduced customer-connectable part kits and telephones, sold through PhoneCenter stores in the early 1970s. For this purpose, Illinois Bell started installing modular telephone sets on a limited scale in June 1972. The patents by Edwin C. Hardesty and coworkers, US 3699498  (1972) and US 3860316  (1975), followed by other improvements, were the basis for the modular molded-plastic connectors that became commonplace for telephone cords by the 1980s. In 1976, these connectors were standardized nationally in the United States by the Registration Interface program of the Federal Communications Commission (FCC), which designated a series of Registered Jack (RJ) specifications for interconnection of customer-premises equipment to the PSTN.

Modular connectors have gender: plugs are considered to be male, while jacks or sockets are considered to be female. Plugs are used to terminate cables and cords, while jacks are used for fixed locations on surfaces of walls, panels, and equipment. Other than telephone extension cables, cables with a modular plug on one end and a jack on the other are rare. Instead, cables are usually connected using a female-to-female coupler, having two jacks wired back-to-back.

Most modular connectors are designed with a latching mechanism that secures the physical connection. As a plug is inserted into a jack, a plastic tab on the plug locks against a ridge in the socket so that the plug cannot be removed without disengaging the tab by pressing it against the plug body. The standard orientation for installing a jack in a vertical surface is with the tab down.