The sustainable use of agricultural by-products is vital for circular bioeconomy goals. Sea buckthorn leaves (SBLs), an underused by-product of berry harvesting, show promise for value-added uses. However, their application in beverages is limited due to the lack of eco-friendly extraction methods that preserve heat-sensitive compounds. Pressurised cyclic solid–liquid (PCSL) extraction offers a novel solution, though it has rarely been applied to this matrix. This study aimed to valorise SBL by extracting bioactive compounds using the innovative PCSL method for the development of a functional beverage—kombucha. Five different kombucha samples were prepared using different ratios of SBL extract and green tea: 100% SBL; 75% SBL + 25% green tea; 50% SBL + 50 % green tea; 25% SBL + 75 green tea; 100% green tea. Fermentation was carried out for 10 days, and at different time points, the concentrations of biologically active compounds and the antioxidant effectiveness, antimicrobial activity, and sensory properties of the beverages were evaluated. The kombucha prepared with SBL extract exhibited the highest phenolic compound concentration (1.21 ± 0.03 g_GAE/L) and the least variation in antioxidant activity throughout fermentation (72.83 – 84.75%), while the green tea kombucha showed the lowest phenolic content (0.93 ± 0.03 g_GAE/L) and highest antioxidant changes (43.73 – 86.97%). Microbiological safety evaluation revealed that kombucha containing SBL extract possessed stronger antimicrobial efficacy against the tested pathogens compared to conventional green tea kombucha. Moreover, sensory evaluation demonstrated higher consumer acceptability for kombucha beverages enriched with SBL extracts. These results indicate that SBLs, as an underutilised secondary biomass, represent not only a nutritional resource but also a significant source of bioactive compounds. This valorisation approach highlights the potential of using such biomass in the formulation of functional foods aligned with sustainable and circular economy principles.