Three shipping containers were burnt down at approximately 1am. Two fire engines responded to the scene within five minutes of receiving the call.
No-one was injured, and the fire was out by 2am.
Jordan Reynolds, director of the Guernsey Raiders, suspected the financial damage was ‘easily in the thousands’.
Rugby kits, training gear, games equipment, filing cabinets and office furniture were all destroyed, he said.
Andrea Harris, chairwoman of the Raiders, said the club was in contact with its insurers and still assessing the value of the damage.
‘You don’t think things like this are going to happen in Guernsey until they do,’ Mr Reynolds said. ‘It’s thoroughly disappointing, especially after our battle with Covid.’
Raiders have not played an away match for nearly 18 months as a result of the pandemic.
Miss Harris agreed that the incident was extremely disappointing.
‘Particularly on an island where our community is supposed to be very buoyant and safe. It’s an additional challenge at a time when everyone has been struggling financially with Covid.’
Despite the damage, Mr Reynolds said the Guernsey Raiders team were still training as normal last evening.
‘We can’t stop at this point,’ he said.
‘We’re going to do our best to train and see how we go.’
A report has been filed with police, who confirmed that the incident was being treated as suspicious and investigations are under way.
The club urged anyone with information to come forward.