Given that chicken is one of the most affordable and healthiest sources of protein, its daily consumption continues to rise globally. As a result, approximately 3 billion pounds of chicken feathers are produced worldwide each year, much of which is discarded as waste. These feathers contribute to soil and water pollution and serve as breeding grounds for harmful microorganisms. Consequently, feather waste poses a significant threat to ecosystems and has become a source of environmental pollution. With this in mind, the aim of this work is to develop and use poultry feather waste (PFW) in powdered form as an additive or auxiliary agent combined with a superplasticizer in cementitious materials such as cementitious pastes and cement mortars. An experimental study was carried out in two parts. In the first phase, PFW powder is incorporated into the superplasticizer, which is then added to cementitious pastes at varying dosages. A rheological analysis is conducted to evaluate the effect on flow behavior. In the second phase, the superplasticizer containing PFW powder is applied to cement mortars. This stage focuses on the physical–mechanical characterization of the modified mortars. The results indicate that PFW can be effectively used as a component of superplasticizer formulations in cement mortar systems, enhancing fluidity without compromising the mechanical performance of the material.