The Sedition Act of 1918 (Pub.L. 65–150, 40 Stat. 553, enacted May 16, 1918) was an Act of the United States Congress that extended the Espionage Act of 1917 to cover a broader range of offenses, notably speech and the expression of opinion that cast the government or the war effort in a negative light or interfered with the sale of government bonds.It forbade the use of "disloyal, profane, scurrilous, or abusive language" about the United States government, its flag, or its armed forces or that caused others to view the American government or its institutions with contempt. Those convicted under the act generally received sentences of imprisonment for five to 20 years. The act also allowed the Postmaster General to refuse to deliver mail that met those same standards for punishable speech or opinion. It applied only to times "when the United States is in war." The U.S. was in a declared state of war at the time of passage, the First World War. The law was repealed on December 13, 1920.Though the legislation enacted in 1918 is commonly called the Sedition Act, it was actually a set of amendments to the Espionage Act.

Therefore, many studies of the Espionage Act and the Sedition Act find it difficult to report on the two "acts" separately. For example, one historian reports that "some fifteen hundred prosecutions were carried out under the Espionage and Sedition Acts, resulting in more than a thousand convictions." Court decisions do not use the shorthand term Sedition Act, but the correct legal term for the law, the Espionage Act, whether as originally enacted or as amended in 1918.