The progressive development of digital technologies has enhanced key aspects of smart cities, including healthcare, mobility, civic engagement, and more, providing citizens with a higher quality of life. Despite countless opportunities for citizens, numerous challenges disrupt the balance of urban environments, especially in terms of ecological, economic and social sustainability. Factors such as population growth and the prioritization of urban areas, coupled with the high consumption of resources in city centers, further compromise sustainability, making it an increasing cause for concern. This research is grounded in the theory that smart cities use their implementation of digital solutions to promote sustainable practices. Accordingly, the research reexamines the relationship between key application domains of smart cities and the Sustainable Development Goals (SDGs), exploring technological aspects that enhance the sustainable development of smart cities. Based on the proposed Conceptual Smart-to-Sustainable model, data on the smart cities index and sustainable cities index for European cities were analyzed. While smart and sustainable cities were expected to be strongly correlated, the results revealed only a moderate correlation in certain cases. The strongest correlation was observed with SDG 9 - Industry, Innovation, and Infrastructure, highlighting its significant role in enabling or enhancing the practicality of digital solutions in citizens’ daily lives.