The relations between social trust and political participation is one of the fundamental topics in political sociology. Research indicates that in societies with high levels of social trust also exhibit higher rate of political participation. Social capital is playing a key role in this relationship through the component of trust. Social trust not only facilitates individual interactions but also strengthens social cohesion, trust in institutions, and support for the political system. Onthe contrary, the lack of social trust reduces political participation and collective action. The main question is whether a decline in social trust, particularly trust in political institutions, has had a meaningful correlation with declining political participation in the last three presidential elections. The research findings demonstrate a direct and positive correlation between social trust and political participation; in other words, the level of political participation is a function of citizens’ trust in the political system. To analyze this nexus, political participation in the last Iranian presidential elections of 2017, 2021, and 2024 was examined using Robert Putnam’s and Michael Woolcock’s social capital theories. Results revealed that besides the correlation between social trust and political participation, the imbalance in attention to various types of social capital—bonding, bridging, and linking-are among the most important factors contributing to the decline in social trust. This imbalance has led to decreased citizens’ trust in the system and subsequently reduced their political participation. Such a relationship produces a vicious cycle of declining trust, reduced participation, weakened legitimacy, intensifying crises, and further erosion of trust.