The sandwich chain aims to overtake McDonald's by number of UK outlets in the next four years, aided by an increased focus on advertising.
Its number of outlets in the UK and Ireland is expected to increase by 40% by the end of the year to 700, with a proportionate increase in its media budget. The plan is to open 2000 restaurants by the end of 2010.
In the US, Subway overtook McDonald's two years ago.
Until now, media had been bought by agencies representing regional groups of Subway franchises. With the fast-food chain set to open its 500th UK store next month, it has appointed an agency to oversee a national centralised budget to improve its buying power.
MediaCom, which is Subway's US agency, was appointed without a pitch.
The account will be handled out of its Scottish office.
TDA will continue to handle UK creative, including adapting US TV work for the UK market. Jared Fogle, who lost 17 stone on a Subway diet and stars in most US TV ads, is on a UK promotional tour this week, appearing on Channel 4's Richard & Judy and in the national press.
Six years ago, Subway had just 30 UK outlets. Franchises contribute 3.5% of annual turnover to a marketing pot, so as the number of outlets grows, the media spend grows proportionately.
Deirdre Anderson, chairman of the UK and Ireland franchise advertising board, said there would be synergistic benefits from working with MediaCom, which could bring its experience from working with the brand in the US.