A UI designer plays a critical role in software development by focusing on the visual aspects and user experience of applications. They are responsible for creating intuitive interfaces that enhance user interaction and engagement. This involves designing layouts, selecting color schemes, and ensuring consistency across various elements. UI designers collaborate closely with user experience (UX) designers and developers to implement designs that meet user needs while maintaining functionality. Their work is essential for ensuring that software applications are not only visually appealing but also easy to navigate, contributing to overall user satisfaction and retention.