On Aug. 15, 1971, to prevent further draining of the United States’ gold reserves, President Nixon announced that dollars held by foreign countries would no longer be exchangeable for gold.  The international gold exchange standard that had existed since 1933 was over. The United States dollar was no longer backed by gold in any way and the dollar became a fiat currency supported only by the "full faith and credit" of the United States government.
Televised address by President Nixon on his decision to move away from a gold standard in 1971. Source: "Nixon Ends Bretton Woods International Monetary System," YouTube.com, Aug. 15, 1971