There are two tartans that I know of, the US St. Andrew's tartan, and the American tartan.

www.district-tartans.com/us_standrews.htm

www.district-tartans.com/american.htm

At one time, the US St. Andrew's tartan was known as the Bicentennial tartan, and there was a move to adopt it as the "official" tartan for the 200th anniversary of the US, but that was rejected. The "St. Andrew" in the name refers to the fact that a number of US St. Andrew's societies had adopted it as a tartan for their members to wear.

There are also a number of American state, and in some cases, city and county tartans as well. The District Tartans web site lists them:

http://www.district-tartans.com/usa.htm

Cheers,

Todd