In order to provide healthcare to low-income people, especially those living in rural areas, the Thai government introduced its Universal Coverage Scheme (UCS) in 2001–2002, becoming one of the first low- and middle-income countries to do so. We employ a difference-in-difference-in-difference (DDD) approach to examine whether UCS effectively reduces out-of-pocket healthcare expenditure across income levels and regions. The empirical results note a significant reduction in medical expenditure for the post-UCS period in outpatient as well as inpatient expenditures. The impact of UCS on the difference in medical expenditure between the treated and control groups varies across regions, while there is no significant difference across income level groups. Thus, by helping to pay an affordable copayment for ambulatory care, UCS has overall softened the high medical costs and provides accessible medical resources for poor regions and middle-income people.