BackgroundSocioeconomic inequalities between and within countries lead to disparities in the use of health services. These disparities could lead to child mortality in children under 5 years by depriving them of healthcare. Therefore, initiatives to remove healthcare fees such as the Free Healthcare Initiative (FHCI) adopted in Sierra Leone can contribute to reducing these inequities in healthcare-seeking for children. This study aimed to assess the socioeconomic inequalities in healthcare-seeking for children under 5 years of age before and after the implementation of the FHCI.MethodsData were included on 1207, 2815, 1633, and 1476 children under 5 years of age with fever from the 2008, 2013, 2016, and 2019 nationwide surveys, respectively. Concentration curves were drawn for the period before (2008) and after (2013–2019) the implementation of the FHCI to assess socioeconomic inequalities in healthcare-seeking. Finally, Erreyger’s corrected concentration indices were calculated to understand the magnitude of these inequalities.ResultsBefore the implementation of the FHCI, there were inequalities in healthcare-seeking for children under five (Erreyger’s corrected concentration index (CI) = 0.168, standard error (SE) = 0.049; p < 0.001) in favor of the wealthy households. These inequalities decreased after the implementation of the FHCI (CI = 0.061, SE = 0.033; p = 0.06 in 2013, CI = 0.039, SE = 0.04; p = 0.32 in 2016, and CI = − 0.0005, SE = 0.362; p = 0.98 in 2019). Furthermore, before the implementation of the FHCI, a significant pro-rich inequality in the districts of Kenema (CI = 0.117, SE = 0.168, p = 0.021), Kono (CI = 0.175, SE = 0.078, p = 0.028) and Western Area Urban (CI = 0.070, SE = 0.032, p = 0.031) has been observed. After the implementation of the FHCI in 2019, these disparities were reduced, 11 of the 14 districts had a CI around the value of equality, and only in 2 districts the pro-rich inequality were significant (Western Area Urban (CI = 0.035, SE = 0.016, p = 0.039) and Western Area Rural (CI = 0.066, SE = 0.030, p = 0.027)).ConclusionThe results of this study demonstrated that socio-economic inequalities in healthcare-seeking for children have been considerably reduced after the FHCI in Sierra Leone. To further reduce these inequalities, policy actions can focus on the increase of availability of health services in the districts where the healthcare-seeking remained pro-rich.