The effects of nanoclay on the crystallization behavior, microcellular structure, and mechanical properties of thermoplastic polyurethane (TPU)/clay nanocomposite (TPUCN) foams were investigated using differential scanning calorimetry, rheometry, scanning electron microscope, transmission electron microscopy, and X-ray diffraction. It was found that the nanoclay acted as an effective nucleating agent for both crystal nucleation and cell nucleation. As a result, it significantly enhanced the crystallization behavior of the hard segment (HS) domains in TPU while refining the foamed structure of the microcellular injection molded parts. In particular, the average cell diameter of TPUCN foams decreased from 45 mu m for neat TPU to 27 mu m for TPUCN5 (5 wt% clay) and 18 mu m for TPUCN10 (10 wt% clay). Furthermore, the cell density increased from 0.7 x 10(7) cell/cm(3) for neat TPU to 1.4 x 10(7) cell/cm(3) and 3.1 x 10(7) cell/cm(3) for TPUCN5 and TPUCN10, respectively. In addition, the tensile strength also increased by 56.3% and 89.2% with 5 and 10 wt% clay content, respectively. By controlling the cell nucleation behavior through uniformly dispersed nanoclay, this study demonstrates that it is feasible to produce TPUCN foams via microcellular injection molding with desirable microcellular structures and improved mechanical properties. POLYM. ENG. SCI., 56:319-327, 2016. (c) 2015 Society of Plastics Engineers