Smoke and flames were coming from the roof of a cottage when Muskoka Lakes Township firefighters arrived on scene shortly after 9:30 p.m. on Friday.
The cottage on Stephen Road was gutted.
Crews were able to save three other structures on the property.
The blaze was brought under control within an hour.
No damage estimate and no injuries.
The cause of the fire is under investigation by the OPP and the Ontario Fire Marshal.