In this study, a crude ethanolic extract made of silver maple (Acer saccharinum L.) leaves (SML) was evaluated for antibacterial activity against the phytopathogenic bacteria Clavibacter michiganensis subsp. michiganensis, Pseudomonas syringae pv. tomato, and Xanthomonas fragariae. The extract was shown to cause important inhibition zones against the three bacteria in the disc diffusion assays, revealing its antibacterial activity. The minimal inhibitory concentration (MIC) of the extract was determined thereafter for each bacterium. The extract showed the same MIC value (1.56 mg·mL−1) for the three bacteria. Using a semipreparative high-performance liquid chromatography system, crude ethanolic SML extract was divided in 15 fractions and each fraction was tested for antibacterial activity against X. fragariae with the disc diffusion assay. Among the six fractions causing an inhibition zone, fraction 10 caused the largest inhibition. Fraction 10 was further analyzed by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry. According to quadrupole time-of-flight mass spectrometry, the main peak of fraction 10 was identified as a galloyl-bis-HHDP-glucose (C41H28O26, 936.6454 g·mol−1) isomer, an ellagitannin known for antibacterial activity and for stimulating plant natural defenses. The study opens new avenues of research on the valorization of SML and on the control of plant diseases caused by bacteria in organic and conventional production of horticultural crops.