Active citizenship is one of the defining features of contemporary democracies. Many empirical studies on democratic citizens emphasize their attitudinal and behavioral characteristics, such as critical citizens, monitorial citizens, and self-actualizing citizens. However, few of them deal with how citizens understand ＂citizenship＂ itself. By addressing citizens' understanding of citizenship, this study investigates how the understanding, including civic responsibilities and rights, can inspire political activism, and examines these relationships cross-nationally. The author employs citizenship norms and rights consciousness as individual-level notions of citizenship. These two indicate how people see civic responsibilities and rights, respectively. In terms of citizenship norms, since norms are able to shape role expectations that individuals believe they should satisfy, their content is expected to determine their related behavior. In this sense, duty-related norms can weaken political activism, but engagement-related ones will strengthen political activism. In addition, rights consciousness should encourage political activism, and its different domains may affect the willingness to act separately. It is because that perception represents an awareness of the need to defend one's rights, which would stimulate their willingness to act in politics. As for cross-national differences, it is expected that relationships between citizens' understanding of citizenship and political activism are generally stronger in stable democracies than in new democracies. At the national level, signs of progress in citizenship rights apparently differ between the two groups of countries. At the individual level, having more democratic experience and opportunities leads to citizens being more likely to apply their understanding in actual political activities. The data analyzed are from the 2014 International Social Survey Programme (ISSP). The scope of this study includes 29 democratic countries, and the final pooled sample size is 42,355. The empirical analyses are divided into two parts. This study first conducts several nested multiple regression models to demonstrate the links between political activism and understanding of citizenship. Regarding citizenship norms, civil norms are found to weaken political activism, while both political and social norms have a positive effect on individuals' willingness to act in politics. As for rights consciousness, either the political or civil and social domain is able to increase political activism. The second step involves conducting a comparison between respondents in the stable and new democracies. Using multi-group analysis, this study examines how the relationships differ between stable and new democracies. The empirical results show that except for social norms, all the citizenship norms and rights consciousness have, as expected, a stronger effect on political activism in the stable-democracies samples than in their new-democracies counterparts, whether it is negative or positive. The evidence reflects the fact that with more democratic experience and opportunities, citizens in stable democracies are more likely than those in new democracies to apply their understanding of citizenship in actual political activities. This study demonstrates the importance of understanding of citizenship for affecting political activism and illustrates how these effects differ between stable and new democracies. The results contribute to our understanding of political behavior, democratic deepening, and democratic civic education.