In an effort to overcome issues encountered in additive flame retardants for rigid polyurethane foams (RPUFs), a novel reactive flame-retardant triol (TDHTPP) based on a triazine and phosphate structure was designed and synthesized. This triol was chemically incorporated in the main chains of RPUFs as a chain-extender to prepare inherently flame-retardant RPUFs. TDHTPP showed good solubility in the polyols (polyol 4110 and PEG 400) of RPUF, making it convenient for industrial fabrication, such as in spay foaming. Excellent compatibility of TDHTPP with the polymer matrix endowed flame-retardant RPUFs (FR-RPUFs) much higher compressive strength than that of the neat RPUF, and a low thermal conductivity of ∼0.03 W/(m·K). Notably, with only 5 wt% of TDHTPP incorporated, the resultant RPUF displayed a UL-94 V-0 rating and more importantly, exhibited a great persistent flame retardancy during thermal accelerated aging tests at 140 °C for 96 h. Further characterization revealed that TDHTPP possessed both vapor-phase and condensed-phase flame-retardant behaviors, in which, vapor-phase action was dominant. This work provides a facile route to synthesize RPUFs with persistent flame retardancy, excellent thermal insulation and mechanical properties. Display omitted •A novel reactive retardant THDTPP was designed to prepare inherently flame-retardant RPUFs.•Great compatibility of TDHTPP with matrix endows FR-RPUFs with excellent thermal insulating and mechanical properties.•The persistent flame retardancy of these foams was confirmed by thermal aging tests.•These foams with excellent properties show a promising prospect in the field of building thermal insulation.