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We perform a climatological analysis of summer‐time Arctic cyclone structure in reanalysis data using a phase space approach. A classification scheme for Arctic cyclones is proposed, dependent on whether vorticity structure during development is low‐level‐dominant (LLD) or upper‐level‐dominant (ULD). During growth, LLD cyclones (65.5%) exhibit warm‐core asymmetric structures, whereas ULD cyclones (34.5%) have cold‐core asymmetric structures. LLD cyclones typically have greater thermal asymmetry during growth. However, a transition to a persistent cold‐core axisymmetric structure after maturity is characteristic of summer‐time Arctic cyclones, regardless of structure during growth. LLD cyclones are typically stronger and preferentially track on the Russian coastline where there is high baroclinicity, whereas ULD cyclones tend to be longer‐lived and preferentially track in the Pacific sector, where they can interact with tropopause polar vortices. This study provides a platform for further research into different classes of Arctic cyclones and associated hazardous weather, and ultimately for developing conceptual models. Plain Language Summary: We investigate the structure of cyclones, large‐scale low‐pressure systems, in the summer‐time Arctic, using observations‐based data of the atmosphere. A phase space is used to represent two aspects of cyclone structure, specifically (a) the low‐level horizontal temperature contrast (i.e., whether there is a distinct warm sector), and (b) the vertical change in wind. We classify Arctic cyclones based on their vertical structure as they develop, as either low‐level‐dominant (LLD), where winds decrease with height, or upper‐level‐dominant (ULD), where winds increase with height. During growth, LLD cyclones (65.5%) exhibit warm‐core structures whereas ULD cyclones (34.5%) have cold‐core structures, both with strong low‐level temperature contrasts. LLD cyclones tend to have a stronger warm sector contrast during development. However, after maximum intensity, when a cyclone matures and begins to weaken, both subsets of cyclones typically evolve to have long‐lasting cold‐core structures without a warm sector. LLD cyclones are typically stronger and preferentially track on the Russian coastline, whereas ULD cyclones tend to be longer‐lived, and preferentially track in the Pacific sector of the Arctic. This study provides a platform for further research into different classes of Arctic cyclones, and ultimately for developing conceptual models, which are key for anticipating associated hazardous weather. Key Points: Arctic cyclones in summer are classified by their vorticity structure during growth: low‐level‐dominant (LLD) or upper‐level‐dominant (ULD)LLD cyclones (65.5%) exhibit more warm‐core baroclinic structures, preferentially track on the Russian coastline, and are typically strongerULD cyclones (34.5%) exhibit more cold‐core axisymmetric structures, preferentially track in the Pacific sector, and tend to be longer‐lived [ABSTRACT FROM AUTHOR]