Heat‐resistant materials have made tremendous progress in marine, aerospace and microelectronic fields. Herein, a new class of phthalonitrile resins, branched poly(biphenyl ether triphenyl‐s‐triazine) phthalonitriles, were successfully synthesized via a two‐step, one‐pot reaction, on the basis of 2,4,6‐tris(4‐fluorophenyl)‐1,3,5‐triazine and 4,4′‐biphenol. 4,4′‐Diaminodiphenylsulfone was employed to facilitate the curing reaction, and successful realization of curing behavior was concluded from rheological and differential scanning calorimetric studies, indicating the obtained resins possess favorable processability. The relationship between concentration of reactants and properties of the resins was systematically studied. After thermal curing, the E‐glass fiber‐reinforced composite, prepared with a concentration of reactants of 0.15 g mL−1, shows an admirable glass transition temperature of 480 °C and commendable thermal stability with 5% weight loss temperature in nitrogen of 563 °C, suggesting that the improvement of the thermal properties stems from the branched structure and the phenyl‐s‐triazine units. © 2017 Society of Chemical Industry