Paddy Feeny has been at the DCMS for seven years and is currently director of communications.
He will join Defra in November after the Beijing Olympics handover ceremony, replacing Steve Morris, who joined Portland PR in June.
In his new role, Feeny will handle a team of 120, overseeing public affairs, media relations and internal comms for campaigns such as Act On CO2. Key issues that he will deal with include the revised Kyoto Treaty, which is being negotiated next year.
Feeny has handled PR activities around the BBC, sport and digital, but in the past few years much of the media attention has been focused on the London Olympics.