US President Donald Trump's administration is to make it more difficult for poorer legal migrants to extend their visas or gain permanent resident status (a green card).
The rule targets migrants who rely on public benefits, such as food aid or public housing, for more than a year.
Their applications will be rejected if the government decides they are likely to rely on public assistance in future.
The rule change would reinforce "ideals of self-sufficiency," officials said.
Anyone who relies on one or more publicly funded social safety net benefits, such as Medicaid or food stamps, is known as a "public charge."
According to this rule, an immigrant will be designated as a "public charge" if that person "receives one or more designated public benefits for more than 12 months in the aggregate within any 36-month period."
The new regulation, known as a "public charge rule", was published in the Federal Register on Monday and will take effect on October 15.
Immigrants who are already permanent residents in the US are unlikely to be affected by the rule change.
Undocumented immigrants would not be affected -- unless an avenue opens up for them to apply for green cards or visas since they are largely ineligible for public aid.
It also does not apply to refugees and asylum applicants.
But applicants for visa extensions, green cards or US citizenship will be subject to the change.
Those who do not meet income standards or who are deemed likely to rely on benefits such as Medicaid (government-run healthcare) or housing vouchers in future may be blocked from entering the country.
Those already in the US could also have their applications rejected.
An estimated 22 million legal residents in the US are without citizenship, and many of these are likely to be affected.
Civil rights groups have said the move unfairly targets low-income immigrants. The National Immigration Law Center (NILC) has said it will sue the Trump administration to stop the regulation from taking effect.
But the White House said the current system favors immigrants with family ties rather than those who "are self-sufficient and do not strain our public resources".
Since his inauguration, Mr Trump has cut the number of refugees admitted to the US each year. The White House blocked a Senate compromise immigration proposal in January 2018 in part because it did not include changes to the legal immigration system.
Now the administration is making it more difficult for less affluent individuals to obtain legal US residency - or perhaps even enter the country at all.
This sets up an election clash next year between a president sharpening and broadening his immigration rhetoric and Democrats, many of whom have said they believe all immigrants, legal or otherwise, should be eligible for public aid.
President Trump has made immigration a central theme of his administration. This latest move is part of his government's efforts to curb legal immigration.
"To protect benefits for American citizens, immigrants must be financially self-sufficient," a White House statement read after the rule change was announced.
It said two-thirds of immigrants entering the US "do so based on family ties rather than on skill or merit".
Ken Cuccinelli, the acting director of the US Citizenship and Immigration Service, announced the regulation at a press conference on Monday.
He said finances, education, age, and the level of an applicant's English-language skills will all be considered in green card applications. "No one factor alone" will decide a case, he added.
"We expect people of any income to stand on their own two feet," Cuccinelli said on Monday.
The Trump administration has also cracked down on illegal immigration. Last week, about 680 people were arrested in Mississippi on suspicion of being undocumented migrants.
The number of would-be migrants apprehended at the US southern border with Mexico has been rising over the last two years.
However, the number of undocumented immigrants in the US is falling, according to recent analysis from the Pew Research Center.
In May, President Trump put forward proposals for a new skills-based immigration system, designed to favor younger, better educated, English-speaking workers.
The 837-page rule applies to those seeking to come to or remain in the United States via legal channels and is expected to impact roughly 382,000 people seeking to adjust their immigration, according to the Department of Homeland Security. However, immigration advocates say millions of people could be affected by the regulation.
New York Attorney General Letitia James announced Monday evening that she plans to sue to block the rule.
Immigrant advocates have argued that the rule goes beyond what Congress intended and would discriminate against those from poorer countries, keep families apart and prompt legal residents to forgo needed public aid, which could also impact their US citizen children.
They also said it would penalize even hard-working immigrants who only need a small bit of temporary assistance from the government.
About one in seven adults in immigrant families reported that either the person or a family member did not participate in a non-cash safety net program last year because of fear of risking his or her green card status in the future, an Urban Institute study found.