Harri, political correspondent on the Six O'Clock News, will join Fleishman-Hillard in April, taking the role of senior policy adviser providing ‘board-level counsel on political comms'.
Harri becomes the latest heavyweight journalist to move into PR, following in the footsteps of the former political editor of The Sunday Times, David Cracknell, and ex-News of the World editor Andy Coulson.
Harri was chief political correspondent at BBC News 24 in the run-up to the invasion of Iraq. He was later stationed in New York, but returned to the UK last spring.
Last year, PRWeek revealed that Harri was one name in the frame as the Conservative Party trawled for a heavyweight media operator to oversee the party's communications - a search that ended in the appointment of Coulson.
At the time, Harri said: ‘I have no plans to become a PR man.' This week Harri said his attitude had changed after he was approached by two agencies last year.
‘I looked into the industry and found it far more interesting and intellectually challenging than I imagined,' he said.
Harri also revealed he had been inspired by BT group comms director Peter Morgan, who previously spent 20 years at the BBC before moving to Weber Shandwick.
‘Peter did a very similar move a long time ago,' said Harri.
Hacks to flacks: recent moves
David Cracknell - Was: Political editor, The Sunday Times. Now: Chairman, FD-LLM
Kamal Ahmed - Was: Executive editor, news, The Observer. Now: Group director of comms, Equality and Human Rights Commission
Andy Coulson - Was: Editor, News of the World. Now: Director of communications, Conservative Party
Celia Hall - Was: Medical editor, The Daily Telegraph. Now: Director of media strategy, Santé Communications