JWT partnered with MediaCom and other WPP sister agencies, branding specialist The Brand Union and PR shop Hill & Knowlton, to land the account.
The client, the Corporation for Travel Promotion, is a new body created to take a public/private partnership approach to attracting international visitors to the US.
It has a $200m budget to back what it claims will be America's first ever global marketing campaign, which is expected to launch next year.
Jim Evans, chief executive of CTP, said: "JWT's network is unparalleled, especially within our priority markets. They demonstrated impressive strategic insight and a genuine, shared passion for our mission."
The "Brand USA" strategy will be unveiled at the World Travel Market show in London in November.