The U.S. population has reached 300 million, according to the Census Bureau. Only China and India have larger populations than the United States.
At 1146 UT Tuesday, the U.S. Census Bureau says the 300-millionth U.S. resident arrived.
Demographers say this person was either a newborn, an immigrant crossing the border or someone flying into the United States.
They calculated the moment based on estimates that one baby is born every seven seconds, one person dies every 13 seconds, and a new immigrant arrives in the United States every 31 seconds, adding up to one new American every 11 seconds.
In 1915, the United States reached the 100-million mark. Fifty-two years later, on November 20, 1967, President Lyndon B. Johnson held a news conference to mark the 200th million arrival.
The popular Life magazine crowned a baby boy born in Atlanta, Georgia to a Chinese immigrant mother and American-born father as the 200-millionth American.
Now a 38-year-old attorney and father of three, Robert Ken Woo, Jr., says it was sometimes difficult being known as the 200-millionth American.
"The attention was embarrassing to me," Woo says.
Today's milestone was met with little celebration, and comes at a time when Americans are debating the controversial issue of illegal immigration and environmentalists are warning about the impact a growing population has on the environment.
"It does mean that we are more crowded than we used to be and are using more resources," says Jeffrey Passel, a demographer at the Pew Hispanic Center in Washington.
Joseph Chamie of the New York-based Center for Migration Studies says growth can be both positive and negative.
"More people, more houses; more people, more cars; more people, more plumbers, electricians, more work," he says.
Hispanics are the fastest growing segment of America's population, and demographers say the 300-millionth American could have been born to Hispanic parents or walked across the border from Mexico.