Helen Bower joins Number 10 from her current role as head of the press office at the Foreign and Commonwealth Office's UK representation to the EU, based in Brussels.
Bower replaces Ben Saoul, who is returning to the FCO after two-and-a-half years at Number 10.
She will report to head of news Vickie Sheriff alongside others of the same grade in the Number 10 press office team.
The appointment follows an internal Government trawl.
It is not part of the recent restructure.
Last week, PRWeek revealed that Number 10/Cabinet Office director of policy comms Jenny Grey and the Prime Minister's official spokesman Steve Field notified its press officers of new roles up for grabs following the recent restructure.