The Steel Henge structure was installed at Centenary Riverside, a nature park and flood alleviation scheme in Templeborough, near Rotherham, when it opened last year.
But there are now fears the whole work could be stolen and melted down as scrap and the Wildlife Trust for Sheffield and Rotherham has appealed for help to protect it.
Steel Henge was created from huge blocks of metal excavated from the former steelworks site. It was made up of a 30-metre wide circle of uprights and crossbeams.
It is thought one of the crossbeams was stolen last Wednesday night. Site manager Rob Miller said: 'We're amazed and appalled at the theft of part of the Steel Henge sculpture.
'We're asking people who see anything suspicious or unusual happening on the site to please inform the police immediately. It would be a huge pity to lose any more of the sculpture.'
Picture: Jonathan Gawthorpe.