As a locus of technological innovation, a smart city (SC) is a prototypical city of the future. Moreover, according to scholars, a smart city is also sustainable city. Nonetheless, the environmental aspects of urban sustainability are often de-emphasized in favor of discourses around the technical characteristics of SC technologies. In order to integrate the two, the article introduces the notion of a smart green city (SGC) in which technological means and environmental outcomes are in sustainable balance. SGC is presented here as a unifying concept integrating smart city and green city concepts through the environmental, social, and governance (ESG) framework. To illustrate the positive synergy between a city's greenness and smartness, we derive operational definitions of both based on the online media's attention to the related technology and sustainability initiatives. After calculating the two indices for all the cities worldwide with over one million inhabitants (498 cities), regression analysis is performed to determine the strength and direction of the relationships between a city's greenness and smartness. We find that a city's greenness is positively related to its smartness. Principal component analysis reveals a potential relationship between a city's population and the two indices. In particular, a large city's population negatively affects its greenness but positively affects its smartness. A joint index of smartness and greenness is negatively related to a city's population. Hence, the containment of uncontrolled urban growth is critical for successfully implementing SGC initiatives. The analysis results are of use to policy-makers, city managers, and planners intending to integrate the ESG framework into their future urban development strategies. Moreover, to our knowledge, a joint evaluation of a city's greenness and smartness has never been performed before on the inter-regional level of analysis. Accordingly, such a holistic assessment can be of methodological interest to scholars of smart and sustainable cities.