Transfemoral amputees (TFAs) require higher energy expenditure than able-bodied individuals during walking. In this study we examined the effects on one TFA of training for a month with a portable bilateral hip exoskeleton on (i) energy cost of transport (CoT) and (ii) distance covered during a 6-minute walking test (6mWT), assessed without (NoExo) and with (Exo) the exoskeleton. Results showed that in NoExo the CoT was reduced by 13.4% after the training, while the 6mWT distance increased by 20.3%. However, the CoT increased in Exo compared to NoExo both before (24.8%) and after (11.8%) the training. These results provide initial evidence that robot-mediated training should deserve further exploration as a tool for improving walking efficiency of TFAs. Future research will investigate customized tuning procedures more in depth to maximize the beneficial effects of this approach.