The special relationship and historic links between Sunderland and the capital of America has been celebrated in a unique Agreement of Friendship.
Washington mayor Anthony Williams, left and Sunderland Council's Bob Symonds
The agreement, believed to be the first between Washington DC and a UK city, will develop business links and promote tourism and cultural activities.
It was signed by the leader of Sunderland Council and Washington's Mayor at a ceremony in the US city.
A US flag was also raised over Washington Old Hall in County Durham.
Washington DC is named after the first president of the United States, a descendent of the Washington family, which took its name from Wessyngton, now Washington Village, in County Durham.
More recently, Sunderland has developed strong links with the Eastern seaboard of America, attracting more than ten new US companies to invest in the city and create more than 6,000 jobs.
Cllr Bob Symonds, leader of the authority, said: "We are very proud of our historic link with Washington DC and we have always welcomed visitors from America on the heritage trail in the UK.
"We felt it was time to cement and develop the historic relationship in a way that would take full advantage of the potential benefits for both the people of Sunderland and the people of Washington DC."
Anthony Williams, Mayor of Washington DC, said: "I'm pleased that with the signing of this agreement, our two cities will be formally linked - in history and in deed.
"This agreement marks the beginning of a collaboration that will result in a number of cultural, historical and economic projects.
"We'll see joint projects among educational institutions, new exhibitions in each city on the historical ties, and maybe even an enhanced 4 July celebration in Sunderland with a George Washington celebration in the District."