Software product line (SPL) is a software engineering paradigm for software development. SPL is important in promoting software reuse, leading to higher productivity and quality. A software product within a product line often has specific functionalities that are not common to all other products within the product line. Those specific functionalities are termed “variant features” in a product line. SPL paradigm involves the modeling of variant features. However, little work in SPL investigates and addresses the modeling of variant features specific to UI. UML is the de facto modeling language for object-oriented software systems. It is known that UML needs better support in modeling UIs. Thus, much research developed UML extensions to improve UML support in modeling UIs. Yet little of this work is related to developing such extensions for modeling UIs for SPLs in which variant features specific to user interfaces (UI) modeling must be addressed. This research develops a UML extension, WUIML, to address these problems. WUIML defines elements for modeling variant features specific to UIs for Web-based SPLs. The model elements in WUIML extend from the metaclass and of the UML2.0 metamodel. WUIML integrates the modeling of variant features specific to UIs to UML. For example, in a Web-based patient registration SPL, member products targeting British users may use British date format in the user interface, while member products targeting United States users may use United States date format in the user interface. Thus, this is a variant feature for this product line. WUIML defines a model element, XOR, to represent such exclusive or conditions in a product line user interface model. WUIML would reduce SPL engineers’ efforts needed in UI development. To validate the WUIML research outcome, a case study was conducted. The results of this empirical study indicate that modeling UIs for Web-based SPLs using WUIML is more effective and efficient than using standard UML.