The Internet of Things (IoT) consists of interconnected physical devices and software components. These connected things or objects exchange information in order to provide an end-user service. To fulfil this objective, such applications have to be designed by composing existing objects. However, this is a very difficult task mostly due to the heterogeneity and diversity of available objects. The IoT Composer tool was developed for supporting the development of IoT applications by first providing a behavioural model for objects and their composition. IoT Composer is developed as a web application that provides graphical support for composing available objects. The tool also provides automated validation techniques for verifying that the composition behaves correctly. Finally, a deployment plan is generated and can be run for effectively binding and instantiating all objects involved in the composition. IoT Composer was applied successfully to several real-world case studies. Video URL: https://youtu.be/6Cn3CUm5-qU.