It is well known that rubber-like materials exhibit non-linear behaviour that depends on strain amplitude, ambient temperature, and loading frequency. Comprehensive experimental tests were performed to investigate the non-linear behaviour of a newly developed damping material for structural dampers, ultra-high damping Natural Rubber (UHDNR). The structural dampers currently used to reduce the dynamic response of buildings and similar structures are rarely made from rubber-like materials, but are sometimes made from viscoelastic materials. Compared to a typical viscoelastic material, it has been observed that the UHNDR material is less dependent on loading frequency and ambient temperature but has sufficient damping for application in structural dampers. A rate-dependence is proposed. Good agreement was observed between experimental and analytical results.