Surface-functionalized nitrogen/carbon co-doped polymorphic TiO₂ phase junction nanoparticles uniformly distributed in porous carbon matrix were synthesized by a simple one-step pyrolysis of titanium based metal–organic framework (MOF), NH₂-MIL-125(Ti) at 700 °C under water vapour atmosphere. Introducing water vapour during the pyrolysis of NH₂-MIL-125(Ti) not only functionalizes the derived porous carbon matrix with carboxyl groups but also forms additional oxygen-rich N like interstitial/intraband states lying above the valence band of TiO₂ along with the self-doped carbon, which further narrows the energy band gaps of polymorphic TiO₂ nanoparticles that enhance photocatalytic charge transfer efficiency. Without co-catalyst, sample N-C-TiO₂/C_ArW demonstrates H₂ evolution activity of 426 µmol g_cat^-1 h⁻¹, which remarkably outperforms commercial TiO₂ (P-25) and N-C-TiO₂/C_Ar with a 5-fold and 3-fold H₂ generation, respectively. This study clearly shows that water vapour atmosphere during the pyrolysis increases the hydrophilicity of the Ti-MOF derived composites by functionalizing porous carbon matrix with carboxylic groups, as well as enhancing the electrical conductivity and charge transfer efficiency due to the formation of additional localized oxygen-rich N like interstitial/intraband states. This work also demonstrates that by optimizing the anatase–rutile phase composition of the TiO₂ polymorphs, tuning the energy band gaps by N/C co-doping and functionalizing the porous carbon matrix in the N-C-TiO₂/C nanocomposites, the photocatalytic H₂ generation activity can be further enhanced.