The IBM 3270 is a family of block oriented display and printer computer terminals introduced by IBM in 1971 and normally used to communicate with IBM mainframes. The 3270 was the successor to the IBM 2260 display terminal. Due to the text color on the original models, these terminals are informally known as green screen terminals. Unlike a character-oriented terminal, the 3270 minimizes the number of I/O interrupts required by transferring large blocks of data known as data streams, and uses a high speed proprietary communications interface, using coaxial cable.

IBM no longer manufactures 3270 terminals, but the IBM 3270 protocol is still commonly used via TN3270 clients, 3270 terminal emulation or web interfaces to access mainframe-based applications, which are sometimes referred to as green screen applications.

The 3270 series was designed to connect with mainframe computers, often at a remote location, using the technology then available in the early 1970s. The main goal of the system was to maximize the number of terminals that could be used on a single mainframe. To do this, the 3270 was designed to minimize the amount of data transmitted, and minimize the frequency of interrupts to the mainframe. By ensuring the CPU is not interrupted at every keystroke, a 1970s-era IBM 3033 mainframe fitted with only 16 MB of main memory was able to support up to 17,500 3270 terminals under CICS.

Most 3270 devices are clustered, with one or more displays or printers connected to a control unit (the 3275 and 3276 included an integrated control unit). Originally devices were connected to the control unit over coaxial cable; later Token Ring, twisted pair, or Ethernet connections were available. A local control unit attaches directly to the channel of a nearby mainframe. A remote control unit is connected to a communications line by a modem. Remote 3270 controllers are frequently multi-dropped, with multiple control units on a line.

IBM 3270 devices are connected to a 3299 multiplexer or to the cluster controller, e.g., 3271, 3272, 3274, 3174, using 93 ohm RG-62 coaxial cables in a point-to-point configuration with one dedicated cable per terminal. Data is sent with a bit rate of 2.3587 Mbit/s using a slightly modified differential Manchester encoding. Cable runs of up to 1,500 m (4,900 ft) are supported, although IBM documents routinely stated the maximum supported coax cable length was 2,000 ft (610 m). Originally devices were equipped with BNC connectors, which were later replaced with special dual-purpose connectors (DPCs) supporting the IBM shielded twisted-pair cabling system without the need for red baluns.

In a data stream, both text and control (or formatting functions) are interspersed allowing an entire screen to be painted as a single output operation. The concept of formatting in these devices allows the screen to be divided into fields (clusters of contiguous character cells) for which numerous field attributes, e.g., color, highlighting, character set, and protection from modification, can be set. A field attribute occupies a physical location on the screen that also determines the beginning and end of a field. There are also character attributes associated with individual screen locations.

Using a technique known as "read modified", a single transmission back to the mainframe can contain the changes from any number of formatted fields that have been modified, but without sending any unmodified fields or static data. This technique enhances the terminal throughput of the CPU, and minimizes the data transmitted. Some users familiar with character interrupt-driven terminal interfaces find this technique unusual. There is also a read buffer capability that transfers the entire content of the 3270-screen buffer including field attributes. This is mainly used for debugging purposes to preserve the application program screen contents while replacing it, temporarily, with debugging information.

Early 3270s offered three types of keyboards. The typewriter keyboard came in both a 66 key version, with no programmed function (PF) keys, and a 78 key version with twelve. Both versions had two Program Attention (PA) keys. The data entry keyboard had five PF keys and two PA keys. The operator console keyboard had twelve PF keys and two PA keys. Later 3270s had an Attention key, a Cursor Select key, a System Request key, twenty-four PF keys and three PA keys. There was also a TEST REQ key. When one of these keys is pressed, it will cause its control unit to generate an I/O interrupt to the host computer and present an Attention ID (AID) identifying which key was pressed. Application program functions such as termination, page-up, page-down, or help can be invoked by a single key press, thereby reducing the load on very busy processors.