As of the date this web site was created, American citizens do not need a visa to travel to Argentina, Paraguay or Uruguay. You do, however, need a valid passport. If your plans include visiting Brazil, you will need a Brazilian visa; please contact the consulate nearest you.

Make sure your passport has not expired, leave enough time to renew it if necessary. Renewal by mail makes it easy but it takes some time - don't risk a last minute disappointment.