Andrew MacDougall announced the move on Twitter, posting a link to a statement from his new firm, Publicis Groupe, based in London.
In a letter circulated Wednesday among staff in the Prime Minister's Office, MacDougall — his trademark sense of humour clearly intact — said he has wanted to move to London for some time.
"To my team, thank you for letting me be your boss. On most days you made me look good, except when you screwed up and made yourselves look bad," he wrote.
"Just kidding, I leave this place in your capable hands."
MacDougall's resignation comes at a crucial time for the Harper Conservatives as they grapple with a Senate spending scandal and prepare for what's likely to be a raucous fall sitting of Parliament.
Publicis Groupe has named MacDougall senior executive consultant in its London office. He is expected to leave his current post in early September, just after the G20 summit in Russia, and start his new job in November.
MacDougall also thanked the prime minister's former chief of staff, Nigel Wright, for making the decision to give him the communications director's job.
Wright resigned last month after it was revealed that he gave a $90,000 cheque to Sen. Mike Duffy as a "gift" to pay Duffy's questionable expense claims.
Duffy, along with senators Pamela Wallin, Patrick Brazeau and Mac Harb, are facing RCMP investigations of their expense claims.
MacDougall, 38, began as a PMO spokesperson in 2008, and was named director of communications in April 2012 following the resignation of Angelo Persichilli, who held the job for just seven months.
MacDougall thanked his former boss Kory Teneycke, who "rolled the dice" by bringing him into the PMO with no media experience. He also expressed gratitude for the confidence shown to him by Harper.
"It has been a rare privilege to watch firsthand how the prime minister has led Canada through these tough economic times," he wrote.
"To have been a small part of this endeavour will forever rank as one of my proudest accomplishments."
MSLGroup, Publicis Groupe's strategic communications network, said MacDougall will serve a senior strategist in the development of its London operation.
"Andrew's unique combination of political communication, crisis communication, and digital media experience will bring a fresh perspective to our clients as they navigate an increasingly complex communications environment," said MSLGroup chief executive Jeremy Sice.
"We are pleased to welcome Andrew to our growing team in the United Kingdom."