Sideritis euboea is a Greek plant that is traditionally consumed as a beverage (mountain tea). From in vitro studies, its extract has shown antioxidant and estrogenic activities. In our study we used S. euboea as an enriching food factor in order to produce a new functional food, a jelly dessert, in order to explore its antioxidant effects if consumed on a daily basis by healthy subjects. In this placebo-controlled clinical trial, 63 subjects were recruited for a 1-month nutritional intervention. Twelve subjects were excluded. The remaining 51 subjects were randomly classified in the intervention group (daily consumption of the jelly containing 0.3 g of S. euboea extract) or the placebo group (daily consumption of the same jelly without the enrichment). Vitamins C, A, and E, glutathione, coenzyme Q10, total nitrites, nitrates, total nitrogen oxide, nitrites/nitrates ratio, and total antioxidant status were measured in blood samples before and after the intervention. After the intervention, free glutathione and coenzyme Q10 increased, and nitrites decreased significantly in both groups. The other antioxidant markers were not altered. No statistical significant differences were observed between the two groups. The daily consumption of the functional food, for 30 days, had no effects on the antioxidant status of healthy volunteers.