The first contingent of 200 United States marines has arrived in Darwin.
The troops are there on a six-month rotational basis and will take part in training exercises with the Australian Defence Force.
The two countries are boosting defence ties, with the US eventually deploying a 2,500-strong force in northern Australia by 2017.
The move has irked China but US and Australian leaders have stressed it is not an attempt to contain China.
Australia's Defence Minister Stephen Smith welcomed the troops, and told ABC News that the mood in northern Australia was ''one of very strong enthusiasm''.
In a joint statement, Mr Smith, Prime Minister Julia Gillard and Northern Territory Chief Minister Paul Henderson said the deployment was ''an evolution of existing exercises and activities'' that the defence forces of both countries already engaged in.
This, they added, was the latest chapter in a 60-year security relationship with the US.
"There are no US military bases in Australia, and this will not change," they said.
'Long term' prospects
Last week, Australia played down reports that it planned to allow a US air base on islands in the Indian Ocean.
A US newspaper reported the plans, saying this would be a strategic point to fly spy planes over the South China Sea.
The Washington Post report said that the two countries were planning ''a major expansion of military ties'', including plans for drone flights from the Cocos Islands - a pair of coral atolls in the Indian Ocean north-west of Australia.
However, Australian Defence Minister Stephen Smith described the plans as ''long-term'' and ''down the track''.
The troops deployment in northern Australia was announced by Ms Gillard in November 2011 when US President Barack Obama visited Canberra.
It was seen as a move to counter China's growing influence in the region and likely to bolster US allies in the South China Sea dispute.
China, the Philippines, Vietnam, Malaysia, Brunei and Taiwan have overlapping territorial claims in the area believed to contain rich reserves of oil and natural gas, as well as key shipping lanes.
Canberra and Washington have been long-time allies, but China is Australia's biggest trading partner.