A free open-source toolbox for the Optimisation of BLEnded Stacking Sequences (OptiBLESS) is presented in this paper. Despite the increasing use of composite materials over the last decades, opti-misation tools for composite structures and stacking sequences remain scarce. While a few proprietary and private software are available for this purpose, there are none in the open source domain where their impact could be greatest. The aim of the developed toolbox presented herein is to provide an accessible and easy-to-use tool for the optimisation of blended stacking sequences. The toolbox capabilities and methodology are addressed in the first part of this paper before being applied to the optimisation of a composite aircraft wing. Results successfully demonstrate the toolbox functionality and performance. The OptiBLESS toolbox is released under a BSD 2-clause license allowing redistribution and use in source and binary forms, with or without modifications. The toolbox, including a user manual and working examples, can be downloaded directly from its GitHub repository: https://github. com/TMacquart/OptiBLESS.