Mark Devane has been appointed head of comms at the trust, the independent governing body of the BBC.
He starts in April and replaces Tina Stowell, now head of corporate affairs at the BBC.
Devane has been at Defra since March 2007, when he joined as chief press officer, environment and climate change.
Previously he was at the Department for Culture, Media and Sport, where he worked as senior press officer, media and tourism and then chief press officer, creative industries and tourism.
A former journalist, Devane has also worked in the Department of Health press office.
The BBC Trust works to ensure that the BBC provides high quality output and protects the BBC's independence.