Background: Cassia abbreviata Oliv is believed to possess many pharmacological activities and has been used traditionally to treat many ailments. In Zambia, it is used by traditional healers and the locals to treat various bacterial infections especially in rural areas where traditional medicine is the first or only line of treatment. However, it’s phytochemical content and activity on Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) has not been documented in Zambia. Objective: To investigate the phytochemical composition and antibacterial activity of Cassia abbreviata Oliv stem bark extract against Escherichia coli and Staphylococcus aureus. Materials and Methods: Ethanol and aqueous crude extracts were derived from Cassia abbreviata Oliv stem bark and subjected to qualitative phytochemical screening using standard procedures. The extracts were then used to test for antibacterial activity against standard cultures of E. coli ATCC 25922 and S. ATCC 25923. Ciprofloxacin (5μg) was used as a positive control. The agar disc diffusion was used to determine the antibacterial activity of C. abbreviata at different concentrations (20, 15, 10, 5 and 1mg/mL). The minimum inhibitory concentration (MIC) and zones of inhibition were measured against the tested microorganisms. Results: The phytochemical screening revealed the presence of alkaloids, tannins, flavonoids, saponins, terpenoids, sterols, and phenols. The ethanolic extract was found to have antibacterial activity against S. aureus but not E. coli, while the aqueous extract had no effect on either pathogen. A minimum inhibitory concentration of 5mg/mL was observed with the ethanolic extract. Ciprofloxacin showed better antibacterial activity against both S. aureus and E. coli compared to the extracts. Conclusion: The ethanolic extract showed a concentration-dependent antibacterial activity against S. aureus while the aqueous extract showed no antibacterial activity.