Gay marriage recently became legal in Alaska, Arizona, Idaho, North Carolina, West Virginia and Wyoming.
The government's announcement follows the US Supreme Court's decision earlier this month to decline to hear appeals from five states that sought to keep their marriage bans in place.
It brings the total number of states with federal recognition of gay marriage to 32, plus Washington, DC Couples married in these states will qualify for a range of federal benefits, including Social Security retirement and veterans' benefits.
The attorney general said the government is working "as quickly as possible" to make sure same-sex married couples in these states receive the "fullest array of benefits" that federal law allows.
The Justice Department also has determined that it can legally recognize gay marriages performed this summer in Indiana and Wisconsin after federal courts declared marriage bans in the states unconstitutional.
Subsequent developments created confusion about the status of those unions, but Holder said the US government will recognize the marriages.