Low photocatalytic efficiency, rapid charge recombination and limited light absorption in wide band gap materials like ZnO severely hinder their practical applications. In order to overcome the above mentioned drawbacks and improve the photocatalytic efficiency, we have developed a series of Pt loaded ZnO-reduced graphene oxide (RGO) nanoheterostructures. The photocatalytic activity of these nanoheterostructures in different compositions, under sunlight irradiation, towards the reduction of 4-nitrophenol has been investigated. The prepared nanoheterostructures showed superior photocatalytic activity and composition having 5 wt% of RGO and 2 wt% of Pt in ZnO (ZPG5) was found to exhibit the highest activity. Control experiments were performed on ZnO-RGO (ZG) nanocomposites without Pt to determine the role of Pt loading. The rate constant of 4-nitrophenol reduction reaction obtained using ZPG5 nanoheterostructure was 0.4203 min^-1, which is about 30 folds higher than the rate constant catalyzed using bare ZnO NR (0.0142 min^-1) and 6 folds higher than ZG5. Strong enhancement in the photocatalytic activity could be evidenced for the Pt loaded ZnO-RGO nanoheterostructures under sunlight irradiation, which could be attributed to improved charge separation, enhanced charge transfer across the Schottky barrier between ZnO and Pt and the high ability of RGO to adsorb 4-nitrophenol. The developed nanoheterostructures can be used as highly efficient sunlight activated heterogeneous photocatalysts for several real life applications.