Washington's allies are waiting nervously for clarity from President Donald Trump's administration after the volatile leader demanded NATO more than double its spending target and vowed to end the war in Ukraine.
Hegseth's two days of talk in Brussels with his counterparts from NATO and Ukraine are part of a flurry of visits to Europe this week by top US officials.
Those will culminate with Vice President JD Vance meeting Ukrainian President Volodymyr Zelensky at a security conference in Munich on Friday.
The Pentagon chief on Wednesday will sit down with an international coalition of Ukraine's backers before huddling with the 31 other defence ministers from NATO Thursday.
Trump's return to the White House has set nerves on edge as he pushes his "America First" agenda and has questioned US security commitments in Europe.
He has already rocked allies by announcing tariffs, and in Denmark's case insisting he wants to take over Greenland.
On both Ukraine and their own defence, Hegseth looks set to deliver the message that Washington expects European nations to do more.
"We're going to have straight talk with our friends," Hegseth said Tuesday as he kicked off his European trip at a US military base in Germany.
"The European continent deserves to be free from any aggression, but it ought to be those in the neighbourhood investing the most in that individual and collective defence."
His combative boss has long accused allies of underpaying and says NATO should ratchet up its defence spending target from the current two percent of GDP to five percent.
That goal appears well out of reach for most -- but NATO chief Mark Rutte is expected to highlight spending increases to show Washington Europe is stepping up.
Spurred on by Russia's grinding war on Ukraine, last year 23 of NATO's 32 members reached the two-percent level.
Given the level of the threat from Moscow there is a clear acceptance that spending must rise and Rutte has already said it could need to go "north" of three percent.
Diplomats say they hope Hegseth will lay out the administration's demands, firing a starting pistol on negotiations for setting a new target at a June summit in the Netherlands.
"The most difficult issue remains defence spending," said one European diplomat at NATO.
"We badly need to understand that without genuine commitment on significant increases of defence budgets on national and EU level we will cause major problems for ourselves."
- 'Own this conflict' -
The other core question is Trump's approach to Ukraine as Kyiv's forces struggle to hold back Russia after nearly three years of all-out war.
Trump has pledged to strike a quick deal to end the conflict, but so far Washington has given few indications about any plan.
Diplomats say they aren't expecting Hegseth to lay out any details of how this will be achieved.
Instead, the Pentagon chief is expected to call on European allies to shoulder more of the burden on arming Ukraine.
"The Europeans have to own this conflict going forward," Trump's national security advisor Mike Waltz told US media.
Washington gave Kyiv tens of billions of dollars in military assistance under former president Joe Biden.
But Trump's administration has yet to provide additional aid despite several billion dollars in remaining budget authorisation carrying over from his predecessor's time in office.
The meeting of Ukraine's supporters will for the first time be chaired by Britain and not the United States.
NATO's European members stress that they have actually spent more on helping Ukraine than the United States.
Several countries -- such as Britain -- are expected to make announcements of fresh military support for Ukraine at the meeting.
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