The US is to make changes to the justice system to give greater legal recognition to same-sex marriages.
In a speech at a gay rights event in New York, Attorney General Eric Holder said the Justice Department had a role in "confronting discrimination".
Benefits will include couples being exempt from having to testify against each other in court, and having equal rights to visits in federal prisons.
Same-sex marriages are currently legal in 17 US states and in Washington DC.
Other states grant some marital benefits to gay couples, while others have banned gay marriage outright.
However, the new ruling will apply to federal matters in all states, whether or not they recognise gay unions, as long as the couple married legally in a state which does.
In his speech on Saturday at a Human Rights Campaign dinner, Mr Holder said he would issue a memorandum on Monday "that will for the first time in history formally instruct all Justice Department employees to give lawful same-sex marriages full and equal recognition to the greatest extent possible under the law".
The instructions would apply "In every courthouse, in every proceeding, and in every place where a member of the Department of Justice stands on behalf of the United States", he said.
Same-sex couples will now be able to file jointly for bankruptcy and have equal entitlement to federal benefits paid to surviving partners of police officers killed on duty.
They will have equal rights to visit a spouse in federal prison, to be allowed out of prison on compassionate grounds to care for a spouse or attend their funeral.
The principle of "spousal privilege" will also apply, allowing them to refuse to testify against each other in court.
Mr Holder said the role of the Justice Department in confronting discrimination "must be as aggressive today" as it was during the civil rights era.
Chad Griffin, president of the Human Rights Campaign, said the announcement would "change the lives of countless committed gay and lesbian couples for the better".
"Today, our nation moves closer toward its ideals of equality and fairness for all," the AFP news agency quoted him as saying.
The move comes after the Supreme Court last year struck down part of a law which denied federal benefits to gay couples.
That ruling only applied to states which do recognise same-sex unions, and opponents said the new measures interpreted it too loosely.
Peter Sprigg of conservative lobby group the Family Research Council told the BBC the move created a conflict between state and federal law.
"It is doing something which the Supreme Court actually said the federal government should not do, which is the federal government putting a thumb on the scale and trying to influence the state's definition of marriage," he said.