ABC talk show Jimmy Kimmel Live! has been fined $395,000 (£326,000) for mimicking a presidential alert on the programme, a US regulator has said.
The show replicated the alert tone three times, which is illegal, during a sketch mocking the warning system.
On the same day the programme aired, 3 October last year, the alert was officially tested nationwide.
As part of the trial, more than 200 million US mobile phones received a test "Presidential Alert" notification.
The alert system, which is designed to warn of major threats such as missile attacks, natural disasters and acts of terrorism, is run by the Federal Emergency Management Agency (Fema).
In the event of a national emergency, US President Donald Trump would be in charge of ordering the agency to activate the warning system.
Wireless Emergency Alerts (WEAs) can be sent by state and local public safety officials, the National Weather Service, the National Center for Missing and Exploited Children as well as the president of the United States. They look like text messages, but are designed to alert people with a unique sound and vibration.
By simulating the alert tone, the Jimmy Kimmel Show! breached broadcasting rules, said the Federal Communications Commission (FCC), which regulates television in the US.
Under FCC rules, broadcasters are barred from mimicking the warning system "to avoid confusion when the tones are used, alert fatigue among listeners, and false activation".
ABC admitted to broadcasting the alert on 3 October 2018, but said it did so under the impression that "use of the tone was permissible".
In a statement, ABC - which is owned by Walt Disney Television - said it takes "regulatory compliance seriously and we are pleased to have resolved this issue".
ABC has agreed not to broadcast it again, the FCC said.
Separately, AMC Networks, Discovery and Meruelo have agreed to pay civil fines for broadcasting the presidential alert in programmes.
The penalties agreed are $104,000 (£86,000) for AMC; $68,000 (£57,000) for Discovery; and $67,000 (£55,000) for Meruelo, the FCC said.
Last October's emergency alert trial was held to test the readiness of the system.
The alert produced a tone and showed a notification saying: "THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed."
The test was mandated under a 2015 law, the IPAWS Modernization Act, that said one must be run at least every three years.