Medical State Assistance is a French public health insurance programme that allows undocumented migrants (UM) to access primary, secondary, and tertiary care services free of user charge, either premium or out-of-pocket. The objective of this study is to assess the effect of Medical State Assistance on access to healthcare services and on usual source of care (USC). We rely on representative data of 1,223 UM attending places of assistance to vulnerable populations in Paris and in the greater area of Bordeaux (France). In this sample, 51% of UM are covered by Medical State Assistance. We use probit and ordinary least square regressions to model healthcare uses of undocumented migrants. The results show that UM covered by Medical State Assistance are more likely to access outpatient healthcare services (by +22.4 percentage points) and less likely to do so on non-governmental organizations (by -6.7 percentage points) than their eligible but uncovered counterpart. Additionally, covered undocumented migrants made 36.9% more medical visits in outpatient healthcare services and 65.4% fewer visits in non-governmental organizations than eligible but uncovered ones. Moreover, covered UM are also more likely to report that primary care services are their USC, in preference to emergency departments and other outpatient care services. UM covered by Medical State Assistance are more likely to consult in outpatient healthcare services.