The genus Atriplex provides species that are used as food and natural remedies. In this work, the levels of soluble phenolic acids (free and conjugated) and flavonoids in extracts from roots, stems, leaves and flowers of the unexplored Atriplex sagittata Borkh were investigated by LC-ESI-MS/MS, together with their antioxidant and antihyaluronidase activity. Phenolic acids were present in all parts of A. sagittata; and were most abundant in the leaves (225.24 μg/g dw.), whereas the highest content of flavonoids were found in the flowers (242.71 μg/g dw.). The most common phenolics were 4-hydroxybenzoic and salicylic acids, kaempferol-3-glucoside-7-rhamnoside, kaempferol-3-rutinoside and the rare narcissoside, which was present in almost all morphotic parts. The stem extract had the highest antioxidant activity and total phenolic content (611.86 mg/100 g dw.), whereas flower extract exerted the most potent antihyaluronidase effect (IC = 84.67 µg/mL; control—quercetin: IC = 514.28 μg/mL). Phytochemical analysis of the flower extract led to the isolation of two triterpene saponins that were shown to be strong hyaluronidase inhibitors (IC = 33.77 and 168.15 µg/mL; control—escin: IC = 307.38 µg/mL). This is the first report on the presence of phenolics and saponins in A. sagittata. The results suggest that both groups of metabolites may contribute to the overall activity of this plant species.