Phytochemical screening and antibacterial activities of seed extract of Citrullus lanatus (watermelon) against one gram-positive organism, Staphylococcus aureus and one gram-negative organism, Escherichia coli was carried out. The seed extracts were screened for the presence of tannins, saponins, terpenoids, steroids, flavonoids, glycosides, alkaloids, amino acids and balsams. The agar well diffusion method was employed to determine the effect of the extracts on the growth of the test organisms. The results of the tests carried out on the seeds of Citrullus lanatus (watermelon) shows the phytochemical constituents of the seed extracts which showed the presence of terpenoids, tannins, balsams, flavonoids, glycosides, alkaloids and amino acids while saponins and steroids were absent. The results also showed that not all concentrations of the aqueous and ethanolic extracts of the sample had total inhibition on the growth of the tested organisms. However, the seed extracts inhibited the growth of Staphylococcus aureus and Escherichia coli at various concentrations. It was observed that for the ethanolic extract, only concentrations at 100% and 50% inhibited the growth of the test organisms and for the aqueous extract, only concentrations at 100% inhibited their growth. The results of this study show that seed extracts of C. lanatus have antibacterial activity and are potent just as standard antimicrobial drugs against gram positive and gram negative bacteria.