The phenolic fraction of a commercial cranberry syrup, which is purported to have good properties for the prevention of urinary diseases, has been thoroughly characterized using HPLC-DAD-TOF-MS. A study of its antibacterial activity has also been carried out. For this purpose a new HPLC-DAD-TOF-MS method using negative and positive ionization modes was developed and it was thus possible to identify 34 different compounds, nine of which have been tentatively characterized for the first time in cranberry syrup. It is also important to highlight that different coumarins in this matrix were also determined, which, to our knowledge, have not been found previously in the cranberry. The phenolic fraction obtained by HPLC-DAD was found to be 5.47 mg/mL. Catechin and procyanidins belonging to flavanols were the family of compounds found at the highest concentrations (2.37 mg/mL); flavonols were at a concentration of 1.91 mg/mL and phenolic-acid derivatives were found at the lowest concentration (0.15 mg/mL). With regard to antibacterial activity, the incubation of Escherichia coli with cranberry syrup was found to reduce surface hydrophobicity as a function of the concentration of the extract.