The Roland TR-909 Rhythm Composer, commonly known as the 909, is a drum machine introduced by Roland Corporation in 1983, succeeding the TR-808. It was the first Roland drum machine to use samples for some sounds, and the first with MIDI functionality, allowing it to synchronize with other devices. Though the 909 was a commercial failure, it influenced the development of electronic dance music genres such as techno, house and acid house.

The TR-909 was designed by Tadao Kikumoto, who had also led development on Roland's previous drum machine, the TR-808, and designed the Roland TB-303 synthesizer. Makoto Muroi was also a chief engineer, the software was developed by Atsushi Hoshiai, and the voice circuits were developed by Yoshiro Oue.

The 909 was the first Roland drum machine to use samples, for its crash, ride and hi-hat sounds. Hoshiai sampled his own drum kit for the cymbals, using a mismatched pair of Paiste and Zildjian hi-hat cymbals. He sampled them in 6-bit and edited the waveform on a computer with a CP/M-80 operating system. Other sounds are generated with analog synthesis. According to a Roland representative, the engineers felt that samples had some disadvantages and so opted for a combination of sampled and analog sounds.

Whereas the 808 is known for its "boomy" bass, the 909 sounds aggressive and "punchy". It has 11 percussion voices and offers sounds for bass drum, snare, toms, rimshot, clap, crash cymbal, ride cymbal and hi-hat (open and closed). It omits the clave, cowbell, maracas, and conga sounds from the 808. The bass has controls for attack and decay. The snare has controls for tone and "snappy", which adjusts the amount of the snare wire sound. As the clap and snare are generated via the same noise source, they produce a phasing effect when played together.

The 909 features a sequencer that can chain up to 96 patterns into songs of up to 896 measures, and offers controls including shuffle and flam. Users can add accents to beats. The 909 was the first Roland drum machine to use MIDI, allowing it to synchronize with other MIDI devices, or to allow sounds to be triggered by an external MIDI controller for wider dynamic range. Older Roland machines can be synchronized via its DIN sync port.

The 909 was released in 1983 and retailed for $1,195 USD, equivalent to $3,773 in 2024. It attracted interest in the industry as the first Roland instrument to use sampled sounds.

In its review, Electronics & Music Maker found the 909 easier to use than the 808 and felt it offered the best analog drum sounds on the market. It concluded that it offered a good combination of analog and sampled sounds and that the addition of MIDI brought the 909 "as up to date as it needs to be". One Two Testing found the 909 "gloriously easy to use", but felt it was overpriced and "still sounds like a drum machine, instead of a machine playing drums ... It lacks the authenticity of real sounds for studio work."

The 909 was a commercial failure, as users preferred the more realistic sampled sounds of competing products such as the LinnDrum. Roland ceased production after one year, having built 10,000 units. Roland changed elements of the 909 in later revisions, correcting problems and adjusting sounds. Some users modify their machines to match sounds from earlier revisions.