Fabrication of 3D mesoporous Ag₂O–ZnO heterojunctions at varying Ag₂O contents has been achieved through poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic F-108) as the structure-directing agent for the first time. The mesoporous Ag₂O–ZnO nanocomposites exhibited a mesoporous structure, which revealed a large pore volume and high surface area. The photocatalytic efficiency over mesoporous Ag₂O–ZnO nanocomposites for tetracycline (TC) compared with that over commercial P-25 and pristine ZnO NPs through the visible light exposure was studied. Mesoporous 1.5% Ag₂O–ZnO nanocomposites indicated the highest degradation efficiency of 100% of TC during 120 min of the visible light exposure compared with 5% and 10% for pristine ZnO NPs and commercial P-25, respectively. The TC degradation rate took place much rapidly over 1.5% Ag₂O–ZnO nanocomposites (0.798 μmol L^–1 min^–1) as compared to either commercial P-25 (0.097 μmol L^–1 min^–1) or ZnO NPs (0.035 μmol L^–1 min^–1). The mesoporous 1.5% Ag₂O–ZnO nanocomposite revealed the highest degradation rate among all synthesized samples, and it was 23 and 8 orders of magnitudes greater than those of pristine ZnO NPs and P-25, respectively. The photoluminescence and transient photocurrent intensity behaviors have been discussed to explore photocatalysis mechanisms. It is anticipated that the present work will contribute some suggestions for understanding other heterojunctions with outstanding behaviors.