Reeves, who has been in his role for two years, is moving to Washington DC to raise his two children with his American wife Erica.
The news reportedly broke during the leaving party for David Cameron's director of strategy Steve Hilton, leaving both the Prime Minister and Deputy Prime Minister without their closest advisers.
A spokesman for the Deputy Prime Minister said: 'Richard has been a tremendous asset to the Liberal Democrat operation in Government over the last two years and we will be very sad to see him leave.'
Reeves was instrumental in the formation of the Liberal Democrats' long-term electoral plan, and has been responsible for drafting Clegg's major speeches.
Reeves, who has written a biography of John Stuart Mill, has helped Clegg redefine modern liberalism in a series of speeches covering the open society, youth, unemployment and employee ownership during 2012.