The club has become a vehicle for chairman Dale Vince, a British green energy industrialist, to push his green agenda, which will now be joined by Bellerin.
Bellerin said: "When I first played Forest Green Rovers - all I knew about them was that it was a long way from London!
"As I heard more about the club and its work, I knew I wanted to meet them and be part of it.
"So many people feel there's no solution to the world's problems, but Forest Green are already doing plenty - and showing others the way.
"I'm so excited to be part of the FGR family. I'll be helping where I can, supporting people who want to change the world for the better."
Vince, chairman of Forest Green since 2010, said: "I like Hector's approach, his personal journey and the things he's trying to do - they make sense.
"I'm looking forward to working with him on FGR and this wider agenda we have a shared interest in."