i-mode (Japanese: iモード, ai-mōdo) is a Japanese mobile internet (distinct from wireless internet) service operated by NTT DoCoMo. Unlike Wireless Application Protocols, i-mode encompasses a wider variety of internet standards, including web access, e-mail, and the packet-switched network that delivers the data. i-mode users also have access to other various services such as: sports results, weather forecasts, games, financial services, and ticket booking. Content is provided by specialised services, typically from the mobile carrier, which allows them to have tighter control over billing.

Like WAP, i-mode delivers only those services that are specifically converted for the service, or are converted through gateways.

In contrast with the Wireless Application Protocol (WAP) standard, which used Wireless Markup Language (WML) on top of a protocol stack for wireless handheld devices, i-mode borrows from DoCoMo proprietary protocols ALP (HTTP) and TLP (TCP, UDP), as well as fixed Internet data formats such as C-HTML, a subset of the HTML language designed by DoCoMo. C-HTML was designed for small devices (e.g. cellular phones) with hardware restrictions such as lower memory, low-power CPUs with limited or no storage capabilities, small monochrome display screens, single-character fonts and limited input methods. As a simpler form of HTML, C-HTML does not support tables, image maps, multiple fonts and styling of fonts, background colors and images, frames, or style sheets, and is limited to a monochromatic display.

i-mode phones have a special i-mode button for the user to access the start menu. There are more than 12,000 official sites and around 100,000 or more unofficial i-mode sites, which are not linked to DoCoMo's i-mode portal page and DoCoMo's billing services. NTT DoCoMo supervises the content and operations of all official i-mode sites, most of which are commercial. These official sites are accessed through DoCoMo's i-mode menu but in many cases official sites can also be accessed from mobile phones by typing the URL or through the use of QR code (a barcode).

An i-mode user pays for both sent and received data. There are services to avoid unsolicited e-mails. The basic monthly charge is typically on the order of JPY¥200–300 for i-mode not including the data transfer charges, with additional charges on a monthly subscription basis for premium services. A variety of discount plans exist, for example family discount and flat packet plans for unlimited transfer of data at a fixed monthly charge (on the order of ¥4,000 per month).

i-mode was launched in Japan on 22 February 1999. The content planning and service design team was led by Mari Matsunaga, while Takeshi Natsuno was responsible for the business development. Top executive Keiichi Enoki oversaw the technical and overall development. A few months after DoCoMo launched i-mode in February 1999, DoCoMo's competitors launched very similar mobile data services: KDDI launched EZweb, and J-Phone launched J-Sky. Vodafone later acquired J-Phone including J-Sky, renaming the service Vodafone live!, although initially this was different from Vodafone live! in Europe and other markets. In addition, Vodafone KK was acquired by SoftBank, an operator of Yahoo! Japan in October, 2006 and changed the name to SoftBank Mobile.

Bandai and Namco launched content for i-mode in 1999. Bandai launched the Dokodemo Aso Vegas service in May 1999, reaching over 1 million paid subscribers by March 2000. In December 1999, Namco launched Namco Station, a mobile site for i-mode.

Since 2003, i-mode center is called CiRCUS, which consists of 400 NEC NX7000 HP-UX servers and occupies 4,600 m² floor space in DoCoMo's Kawasaki office. The operation support system is called CARNiVAL, which is hosted in the Sanno Park Tower.