T-Mobile International AG is a German holding company for Deutsche Telekom AG's various mobile communications subsidiaries outside Germany. Based in Bonn, Germany, its subsidiaries operate GSM, UMTS and LTE-based cellular networks in Europe, the United States, Puerto Rico and the US Virgin Islands. The company has financial stakes in mobile operators in both Central and Eastern Europe.

The T-Mobile brand is present in 11 European countries – Austria, Croatia, Czech Republic, Germany, Hungary, Macedonia, Montenegro, the Netherlands, Poland, Slovakia, and the United Kingdom – as well as the US, Puerto Rico and the US Virgin Islands.

Globally, T-Mobile International subsidiaries have a combined total of approximately 150 million subscribers, making the company the world's third-largest mobile-phone service provider by subscribers and the third-largest multinational after the UK's Vodafone and Spain's Telefónica.

Germany's initial mobile-communications services were radiotelephone systems that were owned and operated by the state postal monopoly, Deutsche Bundespost. It launched the analog first-generation C-Netz ("C Network", marketed as C-Tel), Germany's first true mobile phone network in 1985.