The figure is the lowest daily national tally since the start of the summer. The last time the United States reported fewer than 30,000 cases was June 21.
There were also 267 deaths on Monday, the lowest toll since early July.
U.S. cases have been on a steady decline since a peak of more than 77,000 in mid-July.
Despite the declining numbers, the United States still has more cases than any other nation. Johns Hopkins data shows 6.301 million cases to date and 189,200 deaths.
California, Texas and Florida -- the states with the most cases -- all reported low figures on Monday not seen since early to mid-June, according to worldometers.info.
The declines followed several governors urging residents to maintain vigilance during the holiday weekend.
In some locations over the Labor Day weekend, thousands of Americans gathered in large crowds despite warnings from health officials that such gatherings could lead to a spike in cases.
In Las Vegas, the Strip and the downtown area were busy with tourists, many of whom didn't wear masks. Nevada has issued various guidelines, including distancing and a mask mandate from Gov. Steve Sisolak in June.
"Please help us get through this next weekend," he'd cautioned before the weekend. "We had a major setback after Memorial Day weekend. We had another one after July. This is our third big one of the summer."
In Southern California, which saw record-high temperatures and power outages, beaches were busy over the holiday weekend -- and a Burning Man-themed party in San Francisco that drew hundreds was condemned by Mayor London Breed.
"Over 1,000 people crowded ... to celebrate Burning Man," she tweeted. "This was absolutely reckless & selfish. You are not celebrating. You are putting people's lives at risk. You are putting our progress at risk.
"No one is immune from spreading the virus."