Boris Johnson is attending a major conference in the US aimed at tackling the so-called Islamic State.
Mr Johnson arrived in Washington DC early on Tuesday evening and will attend talks with representatives of some 60 countries.
He is also scheduled to meet senior US officials, including secretary of state Rex Tillerson.
Brexit is also expected to feature during the visit, as the UK attempts to prepare the way for a US trade deal after withdrawal from the EU.
Mr Johnson's visit comes as Britain seeks to play down the controversies over alleged spying.
US press secretary Sean Spicer sparked a diplomatic incident earlier this month when he claimed the British security agency GCHQ had spied on President Trump at Barack Obama's request.
In a rare public comment GCHQ said the claims were "utterly ridiculous nonsense" which should be ignored.
The Foreign Secretary will also chair a United Nations Security Council meeting on security in Somalia, famine and South Sudan, and host a UN event on female empowerment around the world.