The charity said it was thought the fox cub, thought to be about 12 weeks old, had been trapped in the pitch black drain since about Wednesday.
Posting on Facebook, the charity thanked the Southern Water crew, saying they could not have rescued the cub without their help and praising their determination and care, adding “as the saying goes, heroes don’t always wear capes.”
The Southern water team said they were “honestly just delighted we had the skills and gear to bring this to a happy conclusion”.
“We hope Penny won’t be diving into any more drains or pipes!” they added.
After treatment at a local vets the fox is recovering and putting on some weight at the island’s RSPCA centre before being released.