An API developer plays a crucial role in application integration by designing and implementing Application Programming Interfaces (APIs). They create interfaces that allow different software systems to communicate with each other seamlessly, enabling the exchange of data and services. By developing robust APIs, they ensure that various applications can leverage each other's functionality, enhancing interoperability and streamlining workflows. Additionally, they may document these APIs and provide support to developers who implement the integration, thereby facilitating a smoother development process.