The devices were revealed at the Mobile World Congress in Barcelona on 25 February 2018, as the successors to the Samsung Galaxy S8 and S8+.

The phone received generally favorable reviews, with critics mostly noting the enhanced camera and better positioned fingerprint scanner.

A software update retrofitted the Galaxy Note 9 feature of warning the user about flaws in photos such as blinking eyes and blur.

Versions sold in the United States, Canada, China, Hong Kong, Japan, and Latin America come with the Qualcomm Snapdragon 845 SoC instead.

[20] The S9 and S9+ have stereo speakers tuned by AKG, as well as Dolby Atmos surround sound support; furthermore, the S9 and S9+ are two of the few smartphones that still retain the 3.5mm headphone jack.

[22] The S9 and S9+ ships with Android 8.0 "Oreo", with Google Mobile Services and the Samsung Experience user interface and software suite.

It is mainly unchanged over the versions found on the S8 and Note 8, but adds additional features such as new customization options, live language translation in the camera app, and landscape (horizontal) orientation support for the Samsung home screen,[23][21][24] a lack of which the Galaxy S3 has been criticized for, including by Evan Blass, in 2012.

[32] In mid-2018, Samsung published a series of mockery commercials named "Ingenius" against iPhones' lack of the Galaxy S9's functionality, including an audio connector, memory card expandable storage, as well as allegedly inferior network transfer rates and camera performance.

In one of those commercials, an unhappy customer failed to locate a memory card slot on her iPhone, to which the Apple Store employee told her that the device lacks such, and suggested her in one word to rely on cloud storage as storage expansion instead, even though it depends on Internet access to function and has slow transfer rates and high latency compared to memory cards.

The customer stated that she did not "want her stuff up there", but locally, to which the Apple store employee suggested purchasing a model with more internal storage.

[33] John McCann from TechRadar complimented the improved camera and the new location of the fingerprint sensor, but criticized that it was too similar to its predecessor (Galaxy S8) and had limited AR Emoji functionality.

[35] Computerworld gave the phone a positive review and said it was "measurably better than its predecessor", but did note there wasn't a new feature that was particularly "exciting".

[37] CNBC's Todd Haselton said on 8 March that the S9+ was superior to the S9, for reasons such as the larger screen and second camera, as well as more RAM and battery power.

However, he was not amused by the average battery life and the addition of Bixby, also saying that Samsung has a poor history of updating their smartphones.

[44] About a week after the release of the devices, users started experiencing an issue in the touchscreen, where it would fail to register input in any particular area of the screen, even after a factory reset.

Samsung has responded with a statement that they are aware of "a limited number of reports of Galaxy S9/S9+ displaying dark colors differently than intended in certain instances" but has not provided an official fix for it yet.