Streptococcus thermophilus (S. thermophilus) FUA329 metabolized ellagic acid (EA) to urolithin (Uro) A with numerous bioactivities. EA is among the chief phenolics in black tea. In the present study, the effect of the S. thermophilus FUA329 fermentation on the composition and antioxidant bioactivities of black tea were investigated. Tea polyphenol (TP) content, antioxidant activity, physicochemical composition, conversion of EA to Uro A, and changes in polyphenolic compounds during fermentation were determined through HPLC and LC-MS analyses. The TP content of FUA329 fermented black tea decreased significantly within 48 h. Moreover, Uro A, an active substance, was detected in the FUA329-fermented black tea at 0.6799 µM and remained stable at 72 h. In addition, the antioxidant activity of the FUA329-fermented black tea was significantly higher than that of the CGMCC 1.8748-fermented black tea. Various polyphenols such as gallic acid, epigallocatechin gallate, epigallocatechin gallate, catechin hydrate, rutin, and EA were also detected in the FUA329-fermented black tea. After fermentation with S. thermophilus FUA329, black tea had lower TP content and higher antioxidant activity and polyphenolic compounds. This strain could be developed as a probiotic and used to explore the underlying molecular mechanism of EA biotransformation to Uro A.