The League Two side have adopted a number of environmentally-friendly measures, from using electric vehicles, to vegan-only meals for players, staff and supporters.
The club became the world's first professional sports team to be certified carbon-neutral by the United Nations in 2018.
"I was really excited when I first discovered the opportunity to get involved at Forest Green Rovers -- and the brilliant work the club is already doing," Bellerin said in a statement.
"It's important that I invest in things I am passionate about -- and I'm excited to help push football into having a sustainable future.
"Forest Green Rovers prove to clubs who say 'they don't have resources to be sustainable' -- that it is possible. With the new stadium, being carbon neutral and vegan the club is doing some mind-blowing work.
"People have a universal love of football, so there's no better industry to promote sustainability -- and what we can do as football supporters to be more environmentally conscious."
Bellerin, 25, became a vegan in 2017 and committed to planting 3,000 trees for every Arsenal victory in the Premier League once football restarted after the lockdown.
Started with a cause to raise awareness of a global need to plant trees and offset carbon emissions. Got told we wouldn't plant any...— Héctor Bellerín (@HectorBellerin) August 13, 2020
50,000 trees now planted. Thank you to everyone who supported and played a part. @onetreeplanted pic.twitter.com/zfG85J52Al
He has since raised money to plant over 50,000 trees in the Amazon rainforest.
"I became a vegan around three years ago and at first, I just wanted to try it and detox my body," Bellerin added.
"Initially, I thought I'd eat meat as a 'treat' -- but, being vegan just worked for me.
"After a month of being vegan, I felt stronger - and I didn't struggle with ankle injuries anymore. I began to feel more energetic on the pitch -- and I began to notice a real difference."