Rae Stewart joined Defra on Monday. He replaces Mark Popescu, who leaves to join the British Forces Broadcasting Service as head of news.
The British Forces Broadcasting Service provides radio and television programmes for the forces. Popescu is a former editor of the BBC Six O'Clock News.
In the new role, Stewart will also act as official press secretary to environment secretary Caroline Spelman.
Stewart said: 'It's a great department and the press office deals with a wide range of fascinating subjects - covering everything from biodiversity and animal welfare to sustainable food production.
'Added to that, it's a department that is right on the frontline whenever there are floods or major outbreaks of animal diseases, which is why the Prime Minister recently called Defra the fourth emergency service.
'Getting the crisis comms right is absolutely crucial, and there's an experienced press office team that is used to operating at the highest level.'
Since joining the Cabinet Office in 2009, he has acted as head of comms for the Iraq Inquiry, in which he decided to share all public hearings with TV news channels and stream the content live on the internet. This set a new template for transparent comms in future official inquiries.
The news comes as Defra awaits a director of comms to replace Andrew Whyte, who left in September to join the Foreign and Commonwealth Office as director of comms.
Applications for the vacant post of director of comms have been received by Defra and are currently being considered, with interviews being held in the next few weeks.
The role is currently being shared by the directorate's management team.