In this paper, we present a novel algorithm for 3D floorplanning with fixed outline constraints and a particular emphasis on thermal awareness. A computationally efficient thermal model that can be used to guide the thermal-aware floorplanning algorithm to reduce the peak temperature is proposed. We also present a novel white space redistribution algorithm to dissipate hotspot. Thermal through-silicon via (TSV) insertion is performed during the floorplanning process as a means to control the peak temperature. Experimental results are very promising and demonstrate that the proposed floorplanning algorithm has a high success rate at meeting the fixed-outline constraints while effectively limiting the rise in peak temperature.