Nature is a large deposit of active molecules of plant origin, and the resources of the flora are far from being fully inventoried. Today, the world continues to search for plants that can be used as the basis for new and relatively new treatments. In vitro antibacterial activity was evaluated on the MeOH extract of leaves, fruit galls, and essential oil mastic gum of Pistacia terebinthus from Tessala (Western Algeria) against four human pathogenic microorganisms (Escherichia coli, Bacillus subtilis, Pseudomonas aeruginosa, Staphylococcus aureus) using a disc diffusion method. The gall extract revealed a remarkable antimicrobial effect against the tested microorganisms. Strong activity was observed for samples of gall extract against Staphylococcus aureus with inhibition zones of 20 mm. These results suggested that the samples of gall extract of P. terebinthus tested for antimicrobial activity can be listed as bactericides.