In recent years, mycosterols have emerged as potential functional ingredients for the development of sterol-enriched food products and dietary supplements. Agaricus blazei is a mushroom rich in bioactive compounds. For commercial purposes, their fruiting bodies must obey rigid morphological criteria. Those not conforming to these criteria are usually discarded, although this does not mean impairment of their content in bioactives. The aim of the present work was to propose the use of commercially discarded A. blazei fruiting bodies for obtaining an extract rich in ergosterol as a fortifier ingredient for yogurts. For extraction, the Soxhlet technology was used and the highest ergosterol yield (around 12%) was achieved in the 5th cycle, yielding 58.53 ± 1.72 µg of ergosterol per 100 g of mushroom (dry weight). The ergosterol rich extract presented notable antioxidant and antimicrobial properties, besides showing no hepatotoxicity. When added to the yogurts it significantly enhanced their antioxidant properties. Furthermore, it did not significantly alter the nutritional or the individual fatty acid profiles of the final dairy products. Thus, A. blazei fruiting bodies that do not conform to the commercial requirements of the market and are normally discarded could be exploited for obtaining a natural high added-value food additive, following the circular bioeconomy concept.