NATO on Monday appointed Rose Gottemoeller as deputy head of the US-led alliance, the first woman to hold the post, a statement said.
Gottemoeller is a career US diplomat whose nomination to the NATO post in March by President Barack Obama ran into stiff Republican criticism that she was too soft on Russia.
"I am delighted to announce the appointment of Rose Gottemoeller, who will bring to NATO a wealth of experience in international security policy, and in areas such as arms control and relations with Russia," alliance head Jens Stoltenberg said in a statement.
Stoltenberg said the appointment of the first woman to the deputy post was a "milestone" for NATO.
The Ukraine crisis and Russia's 2014 annexation of Crimea have plunged relations into a deep freeze, with NATO responding by launching a major revamp to counter a more assertive Moscow.
Gottemoeller, currently Under Secretary for Arms Control and International Security, will replace Alexander Vershbow, another career US diplomat, in October.