City branding is becoming more and more popular, not only as a field of research, but also as a practice of local governments. Municipalities attempt to introduce this concept into their sustainable transformation process, positioning a city among its stakeholders as, for instance, an eco-oriented, innovative, or smart city. An idea that fosters building a strong and consistent city brand is brand personality. The aim of the paper is to diagnose a city’s brand personality dimensions/traits communicated via posts published on official Facebook accounts of cities and to identify differences occurring within this scope between cities of two selected Central and Eastern European countries—Poland and Ukraine. To achieve this aim, the research involved a content analysis of 1700 posts published by 34 municipalities representing regional capital cities of Poland and Ukraine. On the basis of these study results, the authors have supplemented the Aaker (1997) five-dimensional scale of brand personality with the sixth dimension: Prosocial Attitude, which includes cooperative and bureaucratic traits, to better comply with the approach of cities to problems of their inhabitants and their surrounding environment, which is consistent with the idea of sustainability. Furthermore, the fifth trait has been added to characteristics which make up Aaker’s Excitement dimension. Dubbed as patriotic, it refers to the city’s activities related to the local celebration of historical events, indicating national pride and identification with its place of residence. Bureaucratic and patriotic traits were quite frequently observed in posts published on the official Facebook accounts of Ukrainian cities. Moreover, the research proves that in case of Polish cities the dominating brand personality dimensions are Sincerity and Excitement. Ukrainian cities tend to exercise communication based on the Sincerity and Competence dimension.