(1) Background: Açai berry extract is known for its high content in polyphenols and thus is a promising ingredient for cosmetic antiaging formulations; (2) Methods: In this study, the açai extract was firstly evaluated for its total phenol content (Folin Ciocalteau essay) and antioxidant activity (radical scavenging activity—DPPH; radical cation scavenging capacity—ABTS; ferric reducing antioxidant capacity—FRAP). Next, the açai extract was included in an O/W formulation and again was evaluated

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... for its polyphenol content and antioxidant capacity. The formulation was tested for general characteristics, physicochemical properties and microbial stability. The proliferative effect on human immortalized fibroblasts was evaluated by the MTT essay, while TAC assay served to confirm that fibroblasts are protected from UV irradiation. The irritant potential was verified on 20 volunteers. The study concluded with the assessment of the sensorial characteristics of the cosmetic formulation; (3) Results: The pure açai extract exhibited high polyphenol content and antioxidant activity, and these characteristics were preserved in the O/W formulation as well. The O/W cosmetic formulation proved to be stable under accelerated and normal conditions, and the preservatives were successful in challenging the resistance against microbial contamination. The mean irritant potential was zero in all volunteers, and the cosmetic formulation showed a good sensorial profile; (4) Conclusions: Açai extract is an interesting ingredient for cosmetic antiaging formulations.