A team clearing the Avon Street gasworks found the intact stone from 1819 "deep inside" an old gasholder tank.
The gasworks opened two years after the stone was carved, powering the city's first street lighting.
It is hoped that the stone will go on display at Bristol's M-Shed museum.
Historian professor Russell Thomas described the find as "significant" adding it was one of the "oldest and most important artefacts remaining from the manufactured gas industry".
"Bristol was one of the earliest cities to adopt gas street lighting, and Avon Street, along with other sites, played a critical role in the development of the city in the 19th and 20th Centuries and the war effort in both the first and the second World Wars," he said.
Wales & West Utilities' Sarah Gillard said "everyone on site was very excited when we found the stone".
"Historically, when gasholders were filled in, it was with anything available - including demolition rubble," she added.
"Gasholders and old gasworks sites all across our network and across the country have been cleaned up and I know of some amazing things being found - old lamps, an intact jar of pickled cauliflower and, believe it or not, a lorry - but nothing as historically significant as this.
"It's a bit scuffed from the digger and there's some staining from when it's been buried, but we're not going to be repairing or cleaning it up too much - it all adds to the stone's story."
A near 200-year-old commemorative stone, buried at a former gasworks in Bristol, has been uncovered by engineers clearing the site.