The Robot Framework is a popular and widely used test automation framework that abstracts test case specifications toward natural language specifications. This makes it well suited for implementing high-level test cases, at least as long as the functions provided by Robot can support the intended functionality. For more complicated test cases, custom and often deeply nested functionality specifications are required, and the readability of Robot test cases tends to decrease. We present RobotBT, a library for the Robot framework that addresses these shortcomings by adding support for specifying test cases using behavior trees. Behavior trees are a comprehensive method for specifying complex behaviors based on a control flow model that orchestrates the execution of functionality. We evaluated RobotBT on a test suite for GUI testing from G~DATA CyberDefense AG and interviewed their engineers about the usability, readability, and applicability of RobotBT. Our results show that BTs improve the expressiveness and readability of Robot Framework test cases and are applicable to practical problems.