The measure underscores the determination in what was once the epicenter of the U.S. outbreak to prevent a resurgence of cases emerging elsewhere. While cases are down 5% nationally, they soared last week in Oklahoma, Montana, Missouri and 17 other states.
On average, 1,000 people die a day nationwide from COVID-19 with the death toll now over 157,000 with 4.8 million cases.
"Travelers coming in from those states will be given information about the quarantine and will be reminded that it is required, not optional," de Blasio told a news briefing. He added that, under certain circumstances, fines for not observing the quarantine order could be as high as $10,000.
The Sheriff's Office, in coordination with other law enforcement agencies, will begin deploying checkpoints at major bridge and tunnel crossings into New York City on Wednesday.
"This is serious stuff and it's time for everyone to realize that if we're going to hold at this level of health and safety in this city, and get better, we have to deal with the fact that the quarantine must be applied consistently to anyone who's traveled," de Blasio said.
A fifth of all new cases in New York City are from out-of-state travelers, said Dr. Ted Long, who oversees the city's contact tracing program.
Teams will be deployed at Penn Station in Midtown Manhattan on Thursday, he said, to ensure travelers stop to complete a travel form.