This paper presents a comprehensive study on a novel ultrasound-assisted extraction process for Rosa canina, utilizing both dry and fresh fruits, and explores the potential application of Rosa canina extraction as a natural dye and functional agent for cotton fabrics. The ultrasound-assisted extraction employed different solvents, including distilled water, methanol, and a water/methanol mixture (50/50% v/v), conducted at 60 °C for 60 min. The extracted compounds were characterized through ultraviolet–visible (UV-Vis) spectroscopy and high-performance liquid chromatography with ultraviolet spectroscopy (HPLC-UV) analysis to assess the chemical composition. Textile applications were then performed using bio-mordant chitosan in a pre-mordanting process, and the treated cotton fabrics underwent analysis for surface chemistry and chemical composition using Fourier-transform infrared spectroscopy (FTIR). Untreated and treated fabrics, both with and without mordant, were evaluated for their UV protection and antibacterial properties. Color measurements and dyeability properties of the extractions were also assessed. Furthermore, waste solutions from textile applications were analyzed by UV-Vis spectroscopy to investigate the potential transfer of active compounds to the fabrics. Results indicate that Rosa canina, as a plant-based extract, holds significant potential for sustainable dyeing and functional finishing of cotton fabrics.