There is not a Prime Minister in the United States.
The head of state and head of government in the United States is the US president. Currently, this is Barack Obama.
The executive branch of the US is elected separately from the legislative branch, although they are related by their political parties. It is possible to have a President whose party does not hold a majority in Congress.
Americas' Prime Minister
Each country in North America and South America has its own political structure, with its own head of state. Canada and many other countries with a British parliamentary heritage have prime ministers.