Sodium-doped carbon nitride nanotubes (Na_x -CNNTs) were prepared by a green and simple two-step method and applied in photocatalytic water splitting for the first time. Transmission electron microscopy (TEM) element mapping and X-ray photoelectron spectroscopy (XPS) measurements confirm that sodium was successfully introduced in the carbon nitride nanotubes (CNNTs), and the intrinsic structure of graphitic carbon nitride (g-C₃N₄) was also maintained in the products. Moreover, the porous structure of the CNNTs leads to relatively large specific surface areas. Photocatalytic tests indicate that the porous tubular structure and Na⁺ doping can synergistically enhance the hydrogen evolution rate under visible light (λ > 420 nm) irradiation in the presence of sacrificial agents, leading to a hydrogen evolution rate as high as 143 μmol·h^-1 (20 mg catalyst). Moreover, other alkali metal-doped CNNTs, such as Li_x -CNNTs and K_x -CNNTs, were tested; both materials were found to enhance the hydrogen evolution rate, but to a lower extent compared with the Na_x -CNNTs. This highlights the general applicability of the present method to prepare alkali metal-doped CNNTs; a preliminary mechanism for the photocatalytic hydrogen evolution reaction in the Na_x -CNNTs is also proposed.