Telegram Messenger is a globally accessible freemium, cross-platform, encrypted, cloud-based and centralized instant messaging (IM) service. The application also provides optional end-to-end encrypted chats, popularly known as secret chat and video calling, VoIP, file sharing and several other features. It was launched for iOS on 14 August 2013 and Android on 20 October 2013. The servers of Telegram are distributed worldwide with five data centers in different parts of the world, while the operational center is based in Dubai, United Arab Emirates. Various client apps are available for desktop and mobile platforms including official apps for Android, iOS, Windows, macOS, and Linux (although registration requires an iOS or Android device and a working phone number). There are also two official Telegram web twin apps, WebK and WebZ, and numerous unofficial clients that make use of Telegram's protocol. Telegram's official components are open source, with the exception of the server which is closed-sourced and proprietary.

Telegram provides optional end-to-end encrypted chats. Cloud chats and groups are encrypted between the client and the server, so that ISPs and other third-parties on the network can't access data. Users can send text and voice messages, make voice and video calls, and share an unlimited number of images, documents (2 GB per file), user locations, animated stickers, contacts, and audio files. Users can also follow channels.

In January 2021, Telegram surpassed 500 million monthly active users. It was the most downloaded app worldwide in January 2021 with 1 billion downloads globally as of late August 2021. Research suggests that the Telegram surge in January 2021 was driven partly by the deplatforming of Parler by big-tech firms. In June 2022, Telegram surpassed 700 million monthly active users. The same month, Telegram Premium, an optional paid subscription with several extra features, was introduced.

Telegram has overtaken WhatsApp and Facebook Messenger to become the most popular instant messaging application in Belarus, Moldova, Jordan, Armenia, Azerbaijan, Kazakhstan, Kyrgyzstan, Cambodia, Ethiopia, Russia, and Ukraine.