The addition of a blackthorn branch extract (Prunus spinosa L.) to a gel emulsion system containing microalgal oil was examined in order to obtain a functional ingredient (APG), for use as fat replacer in beef patties. Chromatographic analysis indicated that catechins were the major polyphenols present in the Prunus spinosa L. extract. The antioxidant capacity increased as a result of the extract addition, as shown by the comparison of the gel emulsions, with and without it (APG and AG, respectively). Beef patties containing APG as fat replacer (modified patties) had a lower fat content (5.3% versus 10.75%), doubled the antioxidant activity and the DHA content, and improved the stability against oxidation by reducing the peroxide content more than two fold when compared to control patties. In addition, instrumental color measured by the CIE L*a*b* system showed no significant difference between control and modified raw patties. Moreover, the sensory acceptability of the new formulation was confirmed by a like/dislike hedonic test.