ZnO:Au nanostructures with tunable surface Plasmon band were synthesizedvia co-precipitation method. X-ray diffraction analysis, high resolution transmission electron microscopy and Raman spectra confirmed the hexagonal wurtzite phase for these ZnO:Au nanostructures with preferential growth along the (101) plane. The selective enhancement in the intensity of Raman band due to the excited free electrons of Au nanoparticles confirmed the incorporation of Au in ZnO matrix. Scanning electron microscopic images showed the transformation of morphology of these nanostructures from rod geometry to rose flower and then to marigold flower-like structures with increase in the Au content. Detailed investigations were carried out to understand the role of plasmons present in the ZnO:Au nanostructures on the photocatalytic degradation of sulforhodamine B under sunlight. It is found that ZnO:Au nanostructures with plasmon band in the close approximation of solar maximum (-)550 nm as catalysts exhibit ultra-fast degradation of the dye. This highly efficient photocatalytic activity of these nanostructures is attributed to the electron scavenging action of Au due to its high electronegativity, enhanced absorption of sunlight due to plasmons, the enhanced surface area of ZnO:Au nanostructures and the formation of Schottky barrier between the Au and ZnO interface. The reusability and photostability of these catalysts were tested through repetitive cycles and demonstrated that these nanostructures can form excellent reusable photocatalysts for the degradation of toxic organic waste in water.