Advances in artificial intelligence (AI) offer new opportunities for human–AI cooperation in engineering design. Human trust in AI is a crucial factor in ensuring an effective human–AI cooperation, and several approaches to enhance human trust in AI have been explored in prior studies. However, it remains an open question in engineering design whether human designers have more trust in an AI and achieve better joint performance when they are deceived into thinking they are working with another human designer. This research assesses the impact of design facilitator identity (“human” versus AI) on human designers through a human subjects study, where participants work with the same AI design facilitator and they can adopt their AI facilitator’s design anytime during the study. Half of the participants are told that they work with an AI, and the other half of the participants are told that they work with another human participant but in fact they work with the AI design facilitator. The results demonstrate that, for this study, human designers adopt their facilitator’s design less often on average when they are deceived about the identity of the AI design facilitator as another human designer. However, design facilitator identity does not have a significant impact on human designers’ average performance, perceived workload, and perceived competency and helpfulness of their design facilitator in the study. These results caution against deceiving human designers about the identity of an AI design facilitator in engineering design.