A novel series of diethyl{4-[(4-oxoquinazolin-3(4H)-yl)methyl]-1H-1,2,3-triazol-1-yl}alkylphosphonates 9aa-aj and their respective derivatives substituted at C6 of the quinazolinone moiety with a bromine atom (9ba-bj) or a nitro group (9ca-cj) were synthesized and assessed for the antibacterial activity toward selected Gram-positive and Gram-negative bacteria. Their antifungal activity was also screened. Compound 9ac was found to be the most active against Staphylococcus aureus ATCC 6535 (MIC 0.625 mg/mL, MBC 1.25 mg/mL), phosphonates 9ab-ai showed promising activity against Enterococcus faecalis ATCC 29212 (MIC = 0.625 mg/mL, MBC = 1.25 mg/mL), while compounds 9ac-j appeared the most active toward Pseudomonas aeruginosa ATCC 27853 (MIC = 0.625 mg/mL, MBC = 1.25 mg/mL). Antifungal assays of compounds 9aa-aj, 9ba-bj, and 9ca-cj were conducted on Candida albicans ATCC 10231 and Aspergillus brasiliensis ATCC 16404 and revealed noticeable activity of 9aa-aj (MIC = 1.25 mg/mL).