She was speaking in the West Australian capital, Perth, ahead of today's annual US-Australia ministerial talks on foreign policy and defence.
She's also expressed American support for the growing relationship between Australia and India and endorsed joint naval exercises.
But China, she said, should be encouraged to rise peacefully, as a responsible stakeholder in the region.
Correspondent: David Weber
Speaker: Hillary Clinton, US Secretary of State
DAVID WEBER: The secretary of state said that Perth was sitting on a very strategic part of the planet. Hillary Clinton described it as Australia's gateway to the trade and energy routes that connected the Indian Ocean to the Pacific.
HILLARY CLINTON: These waters are at the heart of the global economy and a key focus of America's expanding engagement in the region - what we sometimes call our pivot to Asia.
We never actually left Asia. We've always been here and been a presence here. We consider ourselves a Pacific power. But in the 21st century it's important that we make absolutely clear we are here to stay.
DAVID WEBER: Mrs Clinton said the United States had adopted a strategic priority of encouraging India to play a greater role in Asian affairs. The US supported India's "Look East" policy.
HILLARY CLINTON: It's exciting to see the developments as the world's largest democracy and a dynamic emerging economy begins to contribute more broadly to the region. It's also important to see the burgeoning relationship between Australia and India.
And we support the "Look West" policy here in Australia and certainly applaud the Australian Government's strategic white paper on Asian policy.
DAVID WEBER: Hillary Clinton said the Obama administration would welcome joint exercises between Australian and Indian naval vessels.
The secretary of state said Australia and the US had strong cultural and commercial links in a rapidly changing region.
HILLARY CLINTON: We look for ways to support the peaceful rise of China, to support China becoming a responsible stakeholder in the international community, and hope to see gradual but consistent opening up of a Chinese society and political system that will more closely give the Chinese people the opportunities that we in the United States and Australia are lucky to take for granted.