It is not possible to identify an official 300-millionth American
The population reached the milestone at 0746 (1146GMT) - a timing based on calculations that factor in birth and death rates and migration.
The bureau's maths suggests that the US gains one person every 11 seconds.
But it is not possible to say if the 300-millionth American was a new-born or crossed one of the US borders.
Correspondents say that there is not expected to be the same hullabaloo as when the figure of 100 million was reached in 1915, or the double century in 1967 when President Johnson gave a speech and newborn Robert Ken Woo Jr was hailed the 200-millionth American by Life magazine.
Today, the population figure is mired in the divisive politics of immigration - a hot-button issue ahead of the 7 November mid-term elections, they say.
The population in the US is the third largest in the world, behind China and India.
According to the Census Bureau, 14% of the current US population is Hispanic, compared to 4% in 1966, and it is projected that a quarter of the population will be Hispanic in 2050.
It is also expected that in the next 50 years there will be more Hispanic births in the US than immigrants.
Michael Replogle, of Environmental Defense, told the Associated Press news agency: "If the population grows in thriving existing communities, restoring the historic density of older communities, we can easily sustain that growth and create a more efficient economy without sacrificing the environment."
Vicky Markham, director of the Center for Environment and Population, said "sprawl has become the most predominant form of land use", with the US becoming a "suburban nation".
"Sprawl is, by definition, more spread out. That of course requires more vehicles and more vehicle miles travelled," she told AP.
Other figures released by the Census Bureau, show how America has been changing since previous population milestones.
- In 1915, immigrant citizens came mostly from Germany; in 1967 from Italy; and in 2006 mostly from Mexico
- The average US family had 4.5 people in 1915, 3.3 in 1967 and 2.6 in 2006
- Some 45.9% of Americans were property owners in 1915. That grew to 63.6% in 1967 and reached 68.9% in 2006
- There were 4.5 million people aged 65 and older in 1915, or 4.5%; 19.1 million in 1967 (9.5%) and 36.8 million in 2006 (12.4%)
- Life expectancy was 54.5 years in 1915, 70.5 years in 1967 and 77.8 years in 2006
- About 23% of women were in the work force in 1915, compared to 41% in 1967 and 58% in 2006
- There were 2.5 million cars in 1915, 98.9 million in 1967 and 237.2 million in 2006
- John and Mary topped the list of most popular names in 1915; Michael and Lisa were favourites in 1967; and Jacob and Emily were preferred in 2006.