London's Jargon PR won the account following a six-way competitive pitch.
The brief is to provide media relations and branding support as the retained agency.
NTT DoCoMo has more than 68 million customers in Japan and generates annual turnover of $22.9bn (£16bn). The company is listed in London and New York, as well as Tokyo, where it is based.
Jargon PR MD Simon Corbett said: "Our campaign will support the business at a number of international conferences and events throughout 2016, starting with Mobile World Congress."
Mobile World Congress begins in Barcelona on Monday, and attracts top innovative tech firms from around the world as well as substantial media interest.