Augmented reality (AR) enables users to overlay new information over the real environment, allowing them to get a clear perception of physical systems or even to interact with them. AR is a powerful and promising tool in many areas, including engineering‐related ones. Therefore, it has aroused growing interest in engineering education. The main objectives of this study are to examine engineering students' feedback and conceptual knowledge gain resulting from using an online AR app. The design and development of the app, named “AR DC circuit puzzle,” was driven by the intention to familiarize students with AR, using a tool that addresses direct current circuits' concepts, a subject belonging to almost all undergraduate engineering syllabi. Through structural equation modeling, the analysis of the relationships between several psychological traits related to the use of the AR app revealed that attitude toward the use of AR is very positive and mostly driven by the value students perceived this app to have in the context of their engineering training. The use of a pre‐ and posttest methodology resulted in large average normalized gains and effect sizes, indicating conceptual knowledge gain. The current worldwide situation triggered the massive use of digital tools; hence, it can be anticipated that in the future, there will be a greater need for online learning resources. This study, involving a large sample of engineering students in three consecutive academic years, shows that the used online tool based on AR can engage students and, with the right strategy, can promote knowledge gain.