OBJECTIVE Several factors have been associated with poor adherence to disease-modifying drugs (DMD). The aim of this study is to evaluate the adherence to DMD in people with multiple sclerosis (PwMS) in Argentina and Ecuador. METHODS A cross-sectional study was performed. The study was carried out between June 2020 and October 2020, and 303 PwMS treated with DMD were included. Patients undergoing immune reconstitution treatments were excluded. Two definitions of DMD adherence were previously determined. Adherence to MS treatments was assessed using the multiple sclerosis treatment adherence questionnaire (MS-TAQ). The logistic regression model was used to evaluate factors related to adherence, and p < 0.05 was considered significant. RESULTS The mean age at study entry for patients was 40.7 ± 11.2 years, 207 (68.3%) were female, and the mean EDSS score was 2.2 ± 1.9. The overall adherence in our sample was 78.1% (79.7% in Argentina and 76% Ecuador, p = 0.23). Patients using infusion therapies significantly more often belonged to the adherent group (p = 0.042). Sharing decision-making (OR = 3.4, 95% CI: 1.7-6.9, p = 0.01), lower EDSS (OR = 0.8, 95% IC: 0.6-0.9, p = 0.004), and lower treatment duration (OR = 0.8, 95% IC: 0.6-0.9, p = 0.001) were independent predictors of adherence in our multivariate model. CONCLUSIONS We found a prevalence of non-adherence similar to that previously reported. Furthermore, new factors associated with lower adherence were identified.