A MIDI controller is an input device and electronic musical instrument which typically converts physical interaction to Musical Instrument Digital Interface (MIDI) information. This information can be sent to a sound module, synthesizer, or sampler, or can be recorded using a music sequencer or digital audio workstation for later playback. A MIDI controller may or may not have a synthesizer or speaker built in, and most rely on external equipment to convert MIDI events into an audio signal and then into audible sound.

Often, MIDI controllers resemble traditional musical instruments. The most common type is the MIDI keyboard, which resembles a keyboard instrument like a piano, but parallels for a range of instruments exist, including wind controllers which resemble wind instruments, guitar-like controllers such as the SynthAxe, and electronic drum kits which mimic acoustic drums. There are also some controllers without acoustic parallels, the most common being MIDI-enabled music sequencers and simple drum pad controllers like the Roland Octapad, Korg PadKontrol and Novation Launchpad.

The most basic controllers transmit only data about the pitch and duration of notes, while more sophisticated devices are capable of sending further parameters, such as velocity and pitch bend. MIDI controllers can be cheaper, more portable and more versatile than full hardware synthesizers, although different types vary greatly in cost, and sending MIDI commands to a digital sampler normally produces a less authentic sound than that of a traditional instrument. MIDI controllers are an example of digital music technology, and are often used by producers of electronic music to play software synthesizers (or hardware synthesizers that lack their own keyboards).

MIDI was primarily designed with musical keyboards in mind, and controllers that are not keyboards were once referred to as "alternate" controllers. The standard has nonetheless proved adaptable, and a wide variety of devices and instruments are now able to generate or respond to MIDI information.

There is no clear definition of a MIDI controller: there are many purpose-built controllers designed to resemble instruments, but many electronic instruments not explicitly designed to be MIDI controllers (such as hardware synthesizers) and other tools such as music sequencers and audio control surfaces can also produce MIDI output. Various devices use MIDI Machine Control (MMC) messages to synchronize playback across hardware, and MIDI Show Control (MSC) commands can even be used to coordinate sound, lighting and pyrotechnics in theatrical productions.

In the narrow sense, most purpose-built musical MIDI controllers feature some prominent interface that the performer presses, touches, strikes or blows to send note information. They are often also equipped with a number of buttons, wheels, knobs, sliders, pedals, or other sensors for sending control change events: these can be used to control parameters such as velocity and pitch bend within other physical devices or software synthesizers, providing another dimension of control for a performer.

There also exist other types of controllers, such as pitch-to-MIDI converters, which analyze the pitch or vibrations of a traditional instrument or a voice and convert it to a MIDI signal in real time. Such devices have included the Roland CP-40 and the Fairlight Voicetracker, although these would usually now be replaced with pitch-tracking software. Specific guitar-to-MIDI interfaces, often attached to a guitar using a special pickup, have included the Shadow SH075, the IVL Pitchrider, and the Roland GI-10.

MIDI controllers are conventionally connected to other devices using a MIDI cable, but an increasing number support USB, which is more versatile and widely used. Software applications still recognize such controllers as MIDI devices, and a USB-equipped controller can normally draw all of the power it needs via a USB connection, removing the need for an AC adapter.