Austin, on his 12th trip to the region as defence secretary, said negotiations had started on a Status of Forces Agreement (SOFA) between the US and Fiji, which would enable the US to "deploy and redeploy forces in support of Fiji".
It would also "help us train with the Fijians on a very routine basis," he said alongside Fiji Prime Minister Sitiveni Rabuka in a press conference live-streamed from the city of Nadi, on Fiji's main island Viti Levu.
The US also pledged $4.9 million to support Fiji's military modernisation and signed a bilateral logistics cooperation deal, according to a defence department statement.
Austin's visit to Fiji was the first by a US Secretary of Defense to the strategically located nation of around 1 million people, comprising an archipelago of 322 islands.
It was the last stop on a trip that also took him to Australia, the Philippines and Laos.
Rabuka said Fiji looked forward "to continuing to work closely with the United States going forward, including with the new incoming Trump administration".
Austin said the close US-Fiji relationship would "endure far beyond transition of administrations".
The high profile visit comes after Fiji this year agreed to maintain a policing co-operation deal with China, amid a tussle for influence between Beijing and Washington in the Asia-Pacific.