A US Fish and Wildlife Service (FWS) official said the agency received new information from Zambia and Zimbabwe that the move would benefit conservation in their countries.
The FWS, under former president Barack Obama, determined in 2015 that importing the trophies would not benefit the species in the two African countries.
The FWS official said the US move will allow the two African countries to include US sport hunting as part of their management plans for the elephants and allow them to put "much-needed revenue back into conservation."
"Legal, well-regulated sport hunting as part of a sound management program can benefit the conservation of certain species by providing incentives to local communities to conserve the species and by putting much-needed revenue back into conservation," the FWS spokesperson said in a statement.
Critics, however, note the restrictions were created by the Obama administration in 2014 because the African elephant population had dropped. The animals are listed in the US Endangered Species Act, which requires the US government to protect endangered species in other countries.
"We can't control what happens in foreign countries, but what we can control is a restriction on imports on parts of the animals," CNN quoted Wayne Pacelle, president and CEO of the Humane Society, as saying.
"Reprehensible behavior by the Trump Admin," tweeted the Elephant Project.
"100 elephants a day are already killed," the group said, adding that, "This will lead to more poaching."
The number of elephants in the wild has plummeted 30 per cent overall between 2007 and 2014, despite large scale conservation efforts. In some places it has dropped more than 75 per cent due to ivory poaching.
In 2016, there were just over 350,000 elephants still alive in the wild, down from millions in the early 20th Century.
US President Donald Trump's sons Donald Jr. and Eric are themselves big game hunters, US media reported.
Several years ago, Trump Jr. was criticised for posting a photo of himself with a dead elephant's severed tail.
Safari Club International, a worldwide network of hunters, cheered the announcement by the Trump administration.
"We appreciate the efforts of the Service and the US Department of the Interior to remove barriers to sustainable use conservation for African wildlife," SCI President Paul Babaz said in a statement.