This study analyze whether the fare-free bus policy (“Support Program for Promoting Public Transportation Use Among the Elderly and Others”) implemented in Chungnam in July 2019 increases the bus-usage probability among policy beneficiaries (elderly individuals aged 75 and older) in Chungnam. A difference-in-differences approach was utilized to empirically analyze the change in the bus-usage probability among residents aged 75 years and older in Chungnam following the implementation of free bus fares. Specifically, the difference in bususage probability before and after the policy was implemented was examined between the treatment group (elderly aged 75 years and older in Chungnam) and control group (elderly aged 75 years and older in other regions or those aged < 75 years in Chungnam). The effectiveness of the policy was evaluated by comparing the changes. For an empirical analysis, data from the “Elderly Status Survey” conducted by the Ministry of Health and Welfare and the Korea Institute for Health and Social Affairs in 2017 and 2023—targeting elderly individuals aged 65 years and older nationwide—were utilized. The results show that the fare-free bus policy implemented in Chungnam in July 2019 for residents aged 75 years and older significantly increased the bus-usage probability among the elderly population in the region as compared with the case for their counterparts in neighboring Chungbuk and Jeonbuk. However, in an analysis limited to Chungnam as the region of study, no statistically significant effect of the policy was observed when the control group was defined as elderly individuals under 75 years of age. Considering the relatively low levels of retirement preparedness of Korea’s elderly population as compared with those other developed countries and mobility limitations that restrict external activities, the fare-free bus policy for residents aged 75 years and older implemented in Chungnam can improve the transportation welfare of vulnerable groups. The empirical results of this study indicate that the effects of such a policy can serve as a basis for the long-term promotion and expansion of related initiatives.