Interposer-based 3D ICs (or known as 2.5D ICs) have been seen as an alternative approach to true 3D stacked ICs, which mount multiple dies on a silicon interposer and route signals between dies by the interconnects in the interposer. However, the floorplan of dies on the interposer and the signal assignment for macro-bumps and TSVs will largely impact the wirelength of the interconnects in a 2.5D IC. Because long interconnects would degrade the performance of 2.5D ICs, the multi-die floorplanning problem and signal assignment problem for 2.5D ICs are critical. This paper presents an enumeration-based algorithm and a network-flow-based algorithm to solve the multi-die floorplanning and signal assignment problems in a 2.5D IC, respectively. Also, to speed up the floorplanning and signal assignment algorithms, several acceleration techniques are proposed. The experimental results reveal that this work can effectively reduce the total wirelength in a 2.5D IC and the acceleration techniques can significantly speed up the proposed algorithms.