Hegseth, speaking on Saturday at the Shangri-La Dialogue in Singapore, Asia's premier forum for defence leaders, militaries and diplomats, said a stronger, more self-reliant network of allies is essential to deter aggression and preserve the balance of power.
"There is rightful alarm regarding China's historic military buildup and the expansion of its military activities in the region and beyond," he said.
"A Pacific dominated by any hegemon would unravel the regional balance of power," Hegseth said.
"No state, including China, can impose its hegemony and hold the security or prosperity of our nation and our allies in question."
The United States expects its Asian allies and partners to increase defence spending to 3.5 per cent of GDP as it pledged a $US1.5 trillion ($A2.1 trillion) investment in its military, the Pentagon chief said.
Hegseth stressed allies want stability, not escalation.
"What they want, and what the United States delivers, is strength that is disciplined, resolve that is steady, and leadership that is confident enough to speak and walk softly while carrying a big stick."
Hegseth also struck a measured tone on US-China ties, saying relations are "better than they have been in many years," with more frequent military-to-military engagement helping to manage tensions.
"We are meeting more frequently with our Chinese counterparts by maintaining open lines of military-to-military communication."
Hegseth echoed President Donald Trump's long-standing demand that allies shoulder more of their own defence costs. Trump has pointedly said European and NATO partners should reduce reliance on Washington.
"The era of the United States subsidising the defence of wealthy nations is over," Hegseth said. "
"We need partners, not protectorates. We don't have a strong alliance unless everyone has skin in the game. No freeloading."
Hegseth praised contributions from allies including South Korea, the Philippines, Australia, Singapore, Malaysia and Thailand, and said Japan was taking concrete steps to bolster its defences.
Tokyo and Washington "must each pull our weight to strengthen the US-Japan alliance," he said.
On the Middle East conflict, Hegseth said the US stands ready to resume strikes on Iran if diplomacy fails, as negotiators from Washington and Tehran work to bridge major differences blocking a deal.
"Our ability to recommence if necessary...we are more than capable," Hegseth said. He added that Trump remains "patient" and is seeking a "strong deal" to ensure Iran does not obtain a nuclear weapon.
Trump on Friday said he would convene advisers in a secure White House setting to make a "final determination" on a proposal to end the Iran war.
Hegseth also pushed back on concerns the conflict would distract from Asia-Pacific priorities.
"We can do two things at one time," he said.
