The Law Library of Congress now provides free, online access to Treaties and Other International Agreements of The United States of America, a publication compiled by Charles Bevans. The 13 volumes in this set include bilateral and multilateral treaties signed by the United States from 1776 to 1949. The set also includes a general index to help in locating relevant treaties.
The Law Library of Congress is also currently working to digitize treaties from 1950 to 1984, while newer treaties can be found on the US Department of State website.
To access the treaties, visit the Law Library of Congress website.