Monolithic hierarchical porous carbon assemblies embedded with mesoporous NaTi2(PO4)3 nanocrystals suitable for flexible anodes in sodium ion batteries (SIBs) are prepared by modified vacuum filtration and annealing. The fabrication involves two-step hetero-assembly induced by electrostatic interactions. In the hierarchical porous carbon assembly, the ultrathin amorphous carbon layers connect the mesoporous NaTi2(PO4)3 nanocrystals with the 3D interconnected carbon networks of reduced graphene oxide and carbon nanotubes. Owing to the synergistic effects rendered by the mesoporous nanocrystals and hierarchical porous carbon networks, the unique monolithic assembly provides continuous 3D pathways for electron/ion conduction and enhances the structural stability during repetitive Na insertion/extraction. As a flexible anode in SIBs, it delivers outstanding room-temperature electrochemical performance with high reversible capacity (125mAhg−1 at 1C), long cycling life (82% capacity retention after 5,000 cycles at 10C), as well as high rate capability (73mAhg−1 at 30C). The flexible monolithic anode which exhibits high Coulombic efficiency and reversible capacity at 0°C and 50°C has large potential in flexible high-performance SIBs.