Dr Richard Dixon, the director of Friends Of The Earth Scotland, said he will vote yes for independence in next year's referendum.
The 49-year-old said: "I am still listening to both sides, but I'm at the stage that an independent Scotland is the better option."
Dixon was born in Ireland, grew up in England, and has lived in Scotland for the past 30 years.
The latest high-profile supporter for independence believes there is more potential to improve environmental issues in an independent Scotland.
Dixon, who had been at World Wildlife Fund Scotland since 2002 - and the wildlife charity's director since 2005 - joined Friends Of The Earth Scotland earlier this year.