A flexible box-type floating breakwater composed of a number of membrane ballasts filled with air, water and sand has been developed for wave protection of various coastal and offshore applications. The combination of ballasts of different materials would eventually result in the same draft for all the tested models, and the performance efficiency of each breakwater model was ascertained by the means of physical modelling. A total of four rectangular breakwater models of different dynamic properties have been developed and tested in regular wave environment in a wave flume. The performance efficiency of the breakwater is determined by the wave transmission and energy dissipation characteristics. For comparison purposes, a control test model with similar dimensions is also tested in the similar test conditions. The proposed model with sand ballast at the bottom has been proven to have a higher wave attenuation capability than the other test models and the performance efficiency is comparable to that of the rigid box-type floating breakwater in some cases. © 2020 IOP Publishing Ltd. All rights reserved.