The poverty rate was 15% in 2011, meaning 46.2 million Americans were in poverty, staying flat after three previous years of increases.
Real median income of households in the country dropped by 1.5%.
The average poverty threshold for a family of four in 2011 was $23,021 (Â£14,300).
Income inequality, as measured by the Gini index, increased by 1.6% in 2011, the first time there has been an annual rise in the index since 1993.
The percentage of Americans lacking health insurance fell to 15.7% from 16.3%, the report also said.
Poverty in the US reached a record high in 2010, at 15.1%.

The facility, based on the town's Crichton estate, is due to open in the next 12 months.
The centre will be used primarily to research ways to improve responses to weather-related emergencies such as flooding.
A report to the council's policy and resources committee will give a full update on progress to date.
Initially the facility will have up to six staff.
Last month, the Scottish government gave assurances that plans to develop the centre remained on track.
It followed concerns from Dumfriesshire Labour MSP Elaine Murray about the pace of progress on the project, since it was first announced in August last year.
A report to the council says it is essential the renewed impetus is maintained.
To that end working groups are being set up, and council and government officials are exploring the best location and office arrangements for the centre.

NHS Digital confirmed so many NHS staff use the search engine that it had started asking them to take a quiz to verify they were "not a robot".
News site the Register reported one NHS Trust had told staff to "use Bing" instead.
Google indicated its systems were designed to spot unusual traffic and were working as intended.
Detecting suspicious traffic from one network can help defeat potential cyber-attacks, such as attempts to try to overwhelm a website.
The BBC understands Google is not deliberately singling out NHS traffic.
A Google spokeswoman said: "Our systems are simply checking that searches are being carried out by humans and not by robots in order to keep web users safe. Once a user has filled out the Captcha [security check], they can continue to use Google as normal."
The NHS is one of the biggest employers in the world, with more than a million members of staff.
An email sent by an NHS system administrator suggested the number of staff using the search engine was "causing Google to think it is suffering from a cyber-attack".
NHS Digital told the Register: "We are aware of the current issue concerning NHS IP addresses which occasionally results in users being directed to a simple verification form when accessing Google.
"We are currently in discussion with Google as to how we can help them to resolve the issue."
NHS Digital was unable to suggest what NHS staff may be searching for using Google.
The percentage of Americans living in poverty is statistically unchanged from 2010's record high, even as household income fell, a US Census report says.
[NEXT_CONCEPT]
Councillors are to be given a progress report on plans to develop a national resilience centre in Dumfries.
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NHS staff using Google's search engine has triggered one of its cybersecurity defences.