Aeolian barchan dunes on Earth and other planets have been widely investigated. Much of the understanding of barchan dune morphodynamics comes from field observations, numerical simulations, and downsized water‐tunnel experiments as well. Many of the evolution of barchan dunes in water‐tunnel experiments are similar to those of aeolian cases, although they have notable differences in scale, sand particle motion and hydrodynamic characteristics. Here, we first review the literature on the local similarities between aeolian and downsized subaqueous barchan dunes, focusing on (a) dune formation, (b) dune morphology, (c) particle‐scale characteristics, and (d) sand/dune emission at horns. A comprehensive description of double‐dune interaction modes is then presented to illustrate the local similarity of barchan dune morphodynamics. Specifically, as the interaction mode undergoes a process of "merging‐splitting‐chasing," the similarity between the interaction modes of aeolian and downsized subaqueous dunes continuously decreases. Furthermore, we summarize the significance and limitations of downsized water‐tunnel experiments for barchan dunes, and highlight the focus for future investigation. Plain Language Summary: There is a possibility to use the rapid evolution of downsized dunes under water to derive the morphodynamics of aeolian (wind‐formed) dunes formed over long periods. Here, we discuss the morphodynamics of aeolian and downsized subaqueous (underwater) barchan dunes and explain their local similarities in dune formation, dune morphology, and sand particle motion. We then study, for the first time, their local similarity through a conceptual chain of double‐dune interaction modes, indicating that the similarity between the morphodynamics of the two types of dunes continuously decreases as the interaction mode changes from "merging" to "splitting" and then to "chasing." We conclude that such local similarity cannot be completely denied simply due to the difference in the modes of sand particle motion (i.e., wind vs. water); instead, the coupling effects of sand particle motion and dune body migration in both environments should be considered and compared, which help achieve the steady‐state barchan dune evolution in a macroscopically similar manner. Furthermore, the morphological, physical and dynamic parameters are expected to be integrated to establish a general scaling law describing local similarities between the morphodynamics of aeolian and downsized subaqueous barchan dunes. Key Points: Local similarities between aeolian barchan dunes and their downsized subaqueous counterparts are reviewedThe two are compared based on the characteristics of individual dunes and on the modes of dune interactionsThe significance, limitations and possible future work of the downsized dune experiments are summarized [ABSTRACT FROM AUTHOR]