Tannins are phenolics found in fruits such as wild Rubus berries. The aim of this study was to determine the profile by LC–ESI–MS/MS and NMR, antioxidant capacity (AOX) of the tannins in two Mexican wild blackberries. Fractions rich in tannins (FRT) were isolated using different solvents and chromatographic resins. The LC–ESI–MS/MS and NMR results showed that catechin and epicatechin ( m / z 289 −1 ), which are precursors of proanthocyanidins, as well as other six oligomeric forms of ellagitannins ( m / z 301 −1 ) were found in R. Liebmannii , whereas in R. Palmeri , only ellagitannins were found, highlighting oligomeric forms of lambertianins and sanguiins ( m / z 783 −1 to 1869 −1 ). The purification pathway made it possible to increase the AOX 19.7 times from fruits to FRT showing strong positive correlations ( R 2  > 0.9) with total phenolic content. This study provided a good assessment of the tannin composition of wild blackberries from the Northwest of Mexico and their bioactivity as a potential edible source.