KaiOS is a mobile Linux distribution for keypad feature phones based on the Firefox OS open-source project.^[6] It is developed by KaiOS Technologies (Hong Kong) Limited; a company based in Hong Kong, whose largest shareholder is Chinese electronics conglomerate TCL Corporation.^[7][1] KaiOS runs on feature phones or wireless home phones made with low-power hardware and low power consumption (and therefore long battery life). KaiOS supports modern connectivity technologies like 4G LTE, VoLTE, GPS, and Wi-Fi. KaiOS runs HTML5-based apps. KaiOS supports over-the-air updates and has a dedicated app marketplace called KaiStore.^[8] Some applications are preinstalled onto the phone, including Facebook and YouTube.^[9] As of 1 April 2020^[update], there are 500+ apps in KaiStore. The mobile operating system is comparatively lightweight on hardware resource usage, and is able to run on devices with just 256 megabytes (MB) of memory.^[10]

The operating system was first released in 2017, and is developed by KaiOS Technologies Inc., a Hong Kong-based^[11] company headed by CEO Sebastien Codeville, with offices in other countries. In June 2018, Google invested US$22 million in the operating system.^[12] That same year, India-based telecom operator Reliance Jio also invested $7 million through its sister concern, Reliance Retail for a 16% stake in the company.^[13][14] In May 2019, KaiOS raised an additional $50 million USD from Cathay Innovation, and previous investors Google and TCL Holdings.^[15]

In market share study results announced in May 2018, KaiOS beat Apple's iOS for second place in India,^[16] while Android dominates with 71%, albeit down by 9%. KaiOS growth is being largely attributed to the popularity of the competitively-priced JioPhone.^[17] In Q1 2018, 23 million KaiOS devices were produced.^[18]

In March 2020, Mozilla and KaiOS Technologies announced a partnership to update KaiOS with a modern version of the Gecko browser engine, and more closely aligned testing infrastructure. This change should give KaiOS four years worth of performance and security improvements and new features, including TLS 1.3, WebAssembly, WebGL 2.0, Progressive Web Apps, new video codecs like WebP, AV1, and modern JavaScript and Cascading Style Sheets (CSS) features.^[19][20]

Devices that are installed with KaiOS include (KaiStore may not be included, if device vendor removed it):^[21]

• Alcatel Go Flip^[22] (known as Cingular Flip 2 on AT&T,^[23] Alcatel MyFlip on TracFone Wireless^[24]) and 3088X
• Advan Hape Online (Indonesia) in partnership with Indosat Ooredoo.
• Reliance Jio's JioPhone, F10Q, F101K, F120B, F220B, F211S, F221S, F250Y, F271I, F30C, F41T, F50Y, F61F, F81E, F90M, LF-2401, LF-2402, LF-2403, LF-2403N, all branded as LYF,^{[25][26][27][28]} JioPhone 2, F300B, F310B, JioPhone Lite, F320B, Prima 4G
• HMD Global's Nokia 8110 4G, 2720 Flip, 800 Tough, 6300 4G, 8000 4G, Nokia 2760 Flip^[29][30][31] and 2780 Flip
• Energizer Energy E220, E220S, E241, E241S, E242S and Hardcase H241, H242, H280S, E282SC
• Doro 7010, 7050, 7060^[32]
• Cat B35^[33]
• Gigaset GL7^[34][35]
• Maxcom MK241, MK281^[36]
• WizPhone WP006, launched in Indonesia in partnership with Google and Alfamart.^[37]
• MTN 3G phone (MTN Smart S 3G)^[38]
• Positivo P70S (Brazil)^[39]
• Multilaser ZAPP (Brazil)^[39]
• Tecno T901^[40]
• IKU V400 4G
• I-Plus I4g
• Lava Captain 11 3G
• Jazz Digit 4G (Pakistan)
• Orange Sanza 2, Sanza XL
• Kitochi 4G Smart
• Vodacom Smart Kitochi (Vida), Vodacom Smart Kitochi (Azumi)
• QMobile 4G Plus (Pakistan)
• GeoPhone T15, T19, T19i (Made in India for Bangladesh)
• Sigma mobile X-Style S3500 sKai (Ukraine)
• Ghia KOX1, GK3G, GQWERTY (Mexico)
• myPhone Up Smart, Up Smart LTE (Poland)^[41]
• Symphony PD1 4G (Bangladesh)
• BKAV C85^[42] (Vietnam) in partnership with Viettel. This is the only KaiOS device to not feature Wi-Fi support
• Viettel V6504,^[43] V1207^[44] (Vietnam). The Viettel V1207 was the first wireless home phone runs KaiOS

All KaiOS devices have T9 keypads, except the Jiophone 2 and Ghia GQWERTY, which have QWERTY keypads.

As of February 2018^[update], KaiOS Technologies has partnered with Mozilla, Airfind, Avenir Telecom (Energizer), Facebook, Google, Bullitt, Doro, HMD Global, Micromax, NXP, Spreadtrum, Qualcomm, Jio, Sprint, AT&T, T-Mobile, and Orange S.A.^[45][9][46]

In March 2020, KaiOS Technologies partnered with Mozilla to bring the modern version of Gecko browser engine to future KaiOS builds.^[19][47]

With the release of the Nokia 8110 4G, an active community (colloquially known as Banana Hackers) around both KaiOS and the phone arose and released the first version of a jailbreak. This gave users the ability to use apps outside KaiStore including old Firefox OS apps on KaiOS devices, as well as flashing their devices with community-created ROMs, such as GerdaOS.^[54][55]

Some models can also be rooted, giving the user further control over the device. ^[56]