Carlton Marsh, which has bagged numerous awards over the decades thanks to tireless reclamation work from volunteers and Barnsley Council, was hit late on Sunday night.
The fire - believed to have occurred at about 9.30pm - saw a crew from nearby Cudworth station attend the Shaw Lane site.
A bird-watching hide, fitted about 30 years ago, was set alight and volunteers responsible for the site’s upkeep - who have bagged a Proud of Barnsley award and multiple accolades from Yorkshire in Bloom in recent years - believe it was a ‘miracle’ it didn’t spread and threaten its wildlife.
Cliff Gorman, who founded the steering group responsible for its transformation, told the Chronicle: “Luckily we have a couple at a nearby cottage whose home overlooks the site, so I received notification from them that there was a fire on Sunday night.
“The fire brigade were there quickly but it’s just so upsetting because it’s a mindless act which achieved nothing.
“Cudworth Dike winds around the site and creates a wet woodland fringe which, together with the delightful scrub, makes this a core refuge for the nationally-endangered willow tit, which has several territories on the site.
“The hide must have sheltered thousands of people over the years - it’s been in its position since March 1993 and the arson attack decimated it.
“However, it could have been much worse - I hope the people responsible are caught.”
The marsh went from being one of the most polluted bodies of water in the county due to the Cudworth Dike tributary’s spillage to becoming a hotbed for wildlife, even going as far as being awarded SSSI accreditation - a site of specific scientific interest - by Natural England.
It is now home to deer and more than 100 species of birds, with peregrine falcons, marsh harriers and buzzards - as well as several varieties of owls - even being spotted.
Fish have also made a return to the marsh’s waterways with mirror carp, perch and chub being recorded due to improving water quality - a far cry from its industrial past.
Coun Charlie Wraith added: “People at the site have put in a lifetime’s work into restoring the habitat, which has won so many awards - rightly so - over the years.
“This is the jewel in Barnsley’s crown with regards to its wildlife and habitat so to have something so serious happen, a pointless thing, is nothing more than wanton vandalism.
“It threatened to undo much of the thousands of hours of voluntary work carried out there to create a wildlife haven.
“More police patrols have to be put in place to deter any repeat occurrence - Carlton Marsh has to be protected.”
Bosses from South Yorkshire Fire and Rescue and South Yorkshire Police confirmed they were treating the incident as a deliberate act - with anyone with information urged to report it via 101 - and PCs and PCSOs from Cudworth station will be including the site on their patrols.
Coun James Higginbottom, cabinet spokesperson for environment and highways, said: “I am incredibly angry that the bird-watching hide at Carlton Marsh was deliberately destroyed in an arson attack on Sunday night.
“The hide had been an integral part of the site for 30 years and provided a wonderful space for volunteers to enjoy the wide array of nature at Carlton Marsh.
“It was also an integral part of successive ‘platinum’ victories in the Yorkshire in Bloom awards, celebrating the work that volunteers do on our sites.
“I would appeal to anyone who has any information about this appalling incident to do the right thing and get in touch with South Yorkshire Police on 101.”