One of the most important features which e-learning tools and environments must possess within the scope of lifelong learning is self-directed learning, which can be considered as a form of self-learning. The aim of this study was to determine, based on the views and recommendations of experts, interface design principles for the development of educational web interfaces that will support the self-directed learning of adults. This descriptive study was conducted with the contribution of 12 academicians specializing in interface design and self-directed learning. Within the scope of the study, new interfaces features were identified based on an evaluation of the literature on interface designs for self-directed learning, and the views of subject experts. Based on the study results, it was determined that interface designs supporting self-directed learning must possess five basic features, which include being user-directed, ensuring variety, being supported by learning analytics, being motivational, and being sharing-oriented.