Competitive inhibition of soybean urease by 11 cyclic β-triketones was studied in aqueous solutions at pH 7.4 and 36°C. This process was characterized quantitatively by the inhibition constant (Ki), which showed a strong dependence on the structure of the organic chelating agents (nickel atoms in urease) and varied from 58.4 to 847 μM. Under similar conditions, the substrate analogue (hydroxyurea) acted as a weak urease inhibitor (Ki = 6.47 mM). At 20°C, competitive inhibition of urease with the ligand of nickel atoms (fluoride anion) was pH-dependent. At pH 3.85–6.45, the value of Ki for the process ranged from 36.5 to 4060 μM. Three nontoxic cyclic β-triketones with Ki values of 58.4, 71.4, and 88.0 μM (36°C) were the most potent inhibitors of urease. Their efficacy was determined by the presence of three >C═O− groups in the molecule and minimum steric hindrances to binding with metal sites in soybean urease.