Chemical screening and antimicrobial activities of stems and roots of Rwandan Urtica massaica Mildbr. (Urticaceae) were investigated. Chemical screening revealed that majority of bioactive secondary metabolites was present at higher concentration in stem than in root. Antimicrobial activities of both stem and root bark methanolic extracts, SBME and RBME, were followed by Agar Disc diffusion and Broth Macro-dilution methods; Inhibition zones, Minimum Inhibition Concentration, MIC and Minimum Bactericidal Concentration, MBC; against the tested microbial strains. Values of inhibition zone for SBME against E. coli were 21.667±1.529 to 28.000±2.851mm. RBME inhibited E. coli from 19.667±1.734 to 25.667±1.023mm. Salmonella sp were fully resistant to RBME. SBME inhibited Salmonella sp from 7.167±0.208 to 24.067±2.503mm but inhibited S. aureus from 17.333±1.335 to 21.667±0.334mm. RBME against S. aureus showed 15.000±0.578, 17.000±0.578 and 20.667±0.334mm inhibition zones respectively. MIC values of SBME were 6.250±1.562mg/mL (E. coli), 10.156±4.752mg/mL (Salmonella sp) and 6.250±1.563mg/mL (S. aureus). MIC of RBME were 7.813±1.563mg/mL (E. coli) and 7.813±1.563mg/mL (S. aureus) though it was found inactive against Salmonella sp. MBC values of SBME were 12.500±3.125mg/mL (E. coli), 19.531±10.156mg/mL (Salmonella sp) and 12.500±3.125mg/mL (S. aureus). MBC values of RBME were 15.625±3.125mg/mL (E. coli) and 15.625±3.125mg/mL (S. aureus). The results indicated greater antimicrobial activities of stem than root of U. massaica, thus corroborating with the fact that majority of bioactive phytochemicals were highly present in SBME than in RBME. U. massaica stem should be considered as natural antimicrobial potential to treat infectious diseases.