Two‐dimensional (2D) graphitic carbon nitride (g‐C3N4) nanosheets show brilliant application potential in numerous fields. Herein, a membrane with artificial nanopores and self‐supporting spacers was fabricated by assembly of 2D g‐C3N4 nanosheets in a stack with elaborate structures. In water purification the g‐C3N4 membrane shows a better separation performance than commercial membranes. The g‐C3N4 membrane has a water permeance of 29 L m−2 h−1 bar−1 and a rejection rate of 87 % for 3 nm molecules with a membrane thickness of 160 nm. The artificial nanopores in the g‐C3N4 nanosheets and the spacers between the partially exfoliated g‐C3N4 nanosheets provide nanochannels for water transport while bigger molecules are retained. The self‐supported nanochannels in the g‐C3N4 membrane are very stable and rigid enough to resist environmental challenges, such as changes to pH and pressure conditions. Permeation experiments and molecular dynamics simulations indicate that a novel nanofluidics phenomenon takes place, whereby water transport through the g‐C3N4 nanosheet membrane occurs with ultralow friction. The findings provide new understanding of fluidics in nanochannels and illuminate a fabrication method by which rigid nanochannels may be obtained for applications in complex or harsh environments. A g‐C3N4 membrane assembled from two‐dimensional g‐C3N4 nanosheets demonstrates good chemical and mechanical stability, as well as ultralow water friction when applied in water purification. The nanosheets contain intrinsic (P1) and artificial (P2) nanopores, as well as self‐supporting spacers (S) formed by adhered unstripped fragments.