A £500 reward is being offered for information on who set fire to the recently refurbished pavilion at Saumarez Park.
Part of the new wooden floor is fire damaged, and the building will have to be fenced off while it's repaired.
Police say it happened over the weekend, or early on Monday morning (27 June).
The Japenese Pavilion has only recently been restored by volunteers.
A remodelled structure was unveiled in September 2021 to mark the Rotary Club of Guernsey's centenary.
The restored pavilion
The old one had been closed for years for safety reasons.
The old pavilion
"Anyone witnesses or people with information regarding what happened is asked to come forwards and speak to officers. Crimestoppers are also offering a £500 reward for any information that leads to an arrest and charge of the culprits." - Guernsey Police.
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