Her busy agenda will include a series of meetings with world leaders, including US President Joe Biden and French President Emmanuel Macron, and her first big speech as prime minister on the international stage at the UN General Assembly in New York.
"Ukraine's victories in recent weeks have been inspirational," Ms Truss said. "Time and time again these brave people have defied the doubters and showed what they can do when given the military, economic and political support they need."
Shadow foreign secretary David Lammy, also attending the assembly, said: "Twelve years of Conservative governments making diplomatic gaffes, announcing plans to break international law, failing to live up to promises on climate action and cutting international aid have damaged relations with the US and left Britain isolated on the global stage.
"After being snubbed by the Biden administration within her first weeks in office, Liz Truss urgently needs to wake up to the damage her reckless approach to foreign policy is doing to the UK's national interest."
She has also said she would temporarily scrap green levies on energy bills to bring down prices for consumers.