Metal isolated single atomic sites catalysts have attracted intensive attention in recent years owing to their maximized atom utilization and unique structure. Despite the success of single atom catalyst synthesis, directly anchoring metal single atoms on three-dimensional (3D) macro support, which is promising to achieve the heterogenization of homogeneous catalysis, remains a challenge and a blank in this field. Herein, we successfully fabricate metal single atoms (Pd, Pt, Ru, Au) on porous carbon nitride/ reduced graphene oxide (C₃N₄/rGO) foam as highly efficient catalysts with convenient recyclability. C₃N₄/rGO foam features two-dimensional microstructures with abundant N chelating sites for the stabilization of metal single atoms and vertically-aligned hierarchical mesostructure that benefits the mass diffusion. The obtained Pd₁/C₃N₄/rGO monolith catalyst exhibits much enhanced activity over its nanoparticle counterpart for Suzuki-Miyaura reaction. Moreover, the Pd₁/C₃N₄/rGO monolith catalyst can be readily assembled in a flow reactor to achieve the highly efficient continuous production of 4-nitro-1,1'-biphenyl through Suzuki-Miyaura coupling.