Phosphonamidates are a class of organophosphorus compounds that can exhibit peculiar flame retardant properties thanks to the P-N synergism in gas and condensed phase. Herein we report the straightforward synthesis of 6-(1,3-dimethylureido)dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DOPO-N^urea) from the commercially available H-phosphinate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The final product was obtained in high yield in a single step by reacting under mild conditions DOPO and 1,3-dimethylurea in the presence of a mild oxidant and triethylamine as base. DOPO-N^urea was characterized by multinuclear NMR, ATR-IR, absorption and emission spectroscopies. The couplings with the ³¹P nucleus resulted particularly useful for the unambiguous assignment of the NMR resonances. The structure and selected spectroscopic features of DOPO-N^urea were simulated by means of density functional theory calculations. Given the presence of a P–N bond between the phosphoryl group and one of the urea nitrogen atoms, of potential interest in the field of sustainable flame retardants, the thermal properties of the new phosporamidate were investigated through differential scanning calorimetry and thermogravimetry. The compound melts without decomposition around 140 °C. At about 150 °C an initial weight loss was observed, compatible with the decomposition of the urea moiety. The flame retardant properties of DOPO-N^urea in bio-based polymers are under current investigation.