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A novel type of phosphorus doped Ti3C2Tx MXene nanosheets (P-Ti3C2Tx) is synthesized via a facile and controllable strategy of annealing MXene nanosheets with the presence of sodium hypophosphite. A combination of theoretical density functional theory calculation and experimental X-ray photoelectron spectroscopy discloses that the doped P atoms are prone to fill into Ti vacancies first due to their lowest formation free energy (ΔGP* = −0.028 eV·A−2) and next to bond with surface terminals on MXene layers (ΔGP* = 0.013 eV·A−2), forming P-C and P-O species, respectively. More importantly, the as-obtained P-Ti3C2Tx is, for the first time, investigated as the electrode material for supercapacitors, demonstrating a significantly boosted electrochemical performance by P doping. As a result, P-Ti3C2Tx electrode delivers a high specific capacitance of 320 F·g−1 at a current density of 0.5 A·g−1 (much higher than 131 F·g−1 for undoped MXene), an ultrahigh rate retention of 83.8% capacitance at 30 A·g−1, and a high cycling stability over continuous 5000 cycles.