This study investigates the combined effect of multi-frequency ultrasound-assisted fermentation on enhancing the functional and nutritional quality of sea buckthorn (Hippophae rhamnoides L.) juice. A defined tri-strain lactic acid bacteria (LAB) consortium; Lactiplantibacillus plantarum, Lacticaseibacillus paracasei, and Lactobacillus helveticus was employed to evaluate its impact on phytochemical enrichment, antioxidant potential, and sensory characteristics. The integrated bioprocess resulted in a substantial increase in total phenolic content (23.8-fold) and total flavonoid content (1.18-fold) compared to the unfermented control. Antioxidant activity showed significant enhancement with DPPH radical scavenging increasing from 88.63 % to 122.73 % and FRAP rising from 100.91 to 566.58 µmol/L. HPLC analysis revealed notable increases in quinic acid (1.29-fold), gallic acid (3.20-fold), and citric acid (7.34-fold …