For products with a myriad of systems, groups of specialised engineers develop entire technical sub-systems, and great effort is needed to integrate these systems for fulfilling the product’s intended properties describing its purposeful behaviour. This way of developing products gets even more complex when using a mass customisation strategy because standard designs (reusable modules) have to be designed to fit a range of products. This product development set-up requires that engineers working in different technical domains collaborate and are able to share information in a unified way. This article presents a visual design tool –the Interface diagram– which aims to support the engineering process of developing modularity in complex product systems. The tool is a model of a product system representing the arrangement of its elements and their interfaces. The tool has similar characteristics to a high-level product architecture model, aiming at supporting integration of technical sub-systems by documenting interfaces and interactions among components from different functional sub-systems and among different physical modules. One of the objectives for using the design tool is to support the activity of decomposing a product system into modules consisting of components developed by different engineering teams. The usefulness of the Interface diagram has been tested in an industrial development project showing positive results of shortening the lead time and minimising rework. Moreover, the Interface diagram has been used in interplay with a broader Product Lifecycle Management system. This allows the product structures from the Interface diagram to be enriched with detailed product documentation like computer-aided design, requirements, view models, design specifications and interface descriptions.