Michael Waltz is a former Green Beret and Florida congressman, who has previously called for Europe to do more to support Ukraine and for the US to pull back its own support in the fight against Russian invasion.
The 50-year-old was a member of the Armed Forces and of Intelligence and Foreign Affairs Committees, and served as a counterterrorism adviser to Vice-President Dick Cheney.
His wife, Julie Nesheiwat, was Mr Trump's homeland security adviser during his first administration.