Census Bureau Director Robert Groves says the figure represents an increase of 9.7 percent over the 2000 U.S. resident population of 281.4 million.
Republican-leaning states will pick up a half dozen House seats thanks to the 2010 census, which found the U.S. population shifting to the South and West.
California is still the most populous state with 37.2 million residents. Wyoming is the least populous with 563,626 residents.
Since the 2000 census, Texas gained the most people, up 4.3 million residents to 25.1 million. Nevada has gained the most residents as a percentage of its 2000 census count, growing 35.1 percent to 2,700,551.
Michigan was the only state with a decline, at 0.6 percent. Puerto Rico's resident population also decreased 2.2 percent to 3,725,789.
Groves says the South and West continued their strong population growth over the last decade.
Groves spoke at a National Press Club briefing announcing 2010 census results. He says the South had the fastest growth since 2000, at 14.3 percent, adding 14.2 million people. The West was close behind at 13.8 percent, adding 8.7 million people.
The Northeast had 3.2 percent growth, adding 1.7 million people, while the Midwest had 3.9 percent growth, adding 2.5 million.
President Barack Obama's spokesman says he doesn't expect the results of the new census to have a "huge practical impact" on national politics.
The spokesman, Robert Gibbs, said he has not seen the new data, which was being released Tuesday. Despite migration to Republican-leaning states in the South and West, Gibbs said he didn't see any reason why both parties would not remain competitive in those regions.
The once-a-decade government count will be used to reapportion the 435 House seats among the 50 states. Texas, a strong GOP state, is expected to pick up House seats.
In 2008, President Barack Obama lost in Texas and most of the other states that are gaining House seats. He carried most of the states that are losing House seats, including Ohio and Pennsylvania. Each House district represents an electoral vote in the presidential election process, meaning the political map for the 2012 election will tilt somewhat more Republican.
For the first time in its history, Democratic-leaning California will not gain a House seat after a census.
Starting early next year, most state governments will use detailed, computer-generated data on voting patterns to carve neighborhoods in or out of newly drawn House districts, tilting them more to the left or right. Sometimes politicians play it safe, quietly agreeing to protect Republican and Democratic incumbents alike. But sometimes the party in control will gamble and aggressively try to reconfigure the map to dump as many opponents as possible.
At 9.7 percent, the U.S. is still growing quickly relative to other developed nations. The population in France and England each increased roughly 5 percent over the past decade, while in Japan the number is largely unchanged, and Germany's population is declining. China grew at about 6 percent; Canada's growth rate is roughly 10 percent.
The declining U.S. growth rate since 2000 is due partly to the economic meltdown in 2008, which brought U.S. births and illegal immigration to a near standstill compared with previous years. The 2010 count represents the number of people - citizens as well as legal and illegal immigrants - who called the U.S. their home on April 1.