Aim. Multimorbidity is a growing phenomenon in primary care, and knowledge of the influence of social determinants on its evolution is vital. The aim of this study was to understand the relationship between multimorbidity and immigration, taking into account length of residence in the host country and area of origin of the immigrant population. Methods. Cross-sectional retrospective study of all adult patients registered within the public health service of Aragon, Spain (N = 1 092 279; 144 238 were foreign-born), based on data from the EpiChron Cohort. Age-standardized prevalence rates of multimorbidity were calculated. Different models of binary logistic regressions were conducted to study the association between multimorbidity, immigrant status and length of residence in the host country. Results. The risk of multimorbidity in foreign-borns was lower than that of native-borns [odds ratio (OR): 0.54, 95% confidence interval (CI): 0.53-0.55]. The probability of experiencing multimorbidity was lowest for Asians (OR: 0.34, 95% CI: 0.31-0.37) and Eastern Europeans (OR: 0.42, 95% CI: 0.40-0.43), and highest for Latin Americans (OR: 0.70, 95% CI: 0.68-0.72). Foreign-born immigrants residing in Aragon for >= 5 years had a higher multimorbidity risk than those residing for < 5 years (OR: 2.3, 95% CI: 2.2-2.4). Conclusion. Prevalence of multimorbidity is lower among foreign-borns as compared with native-borns, but increases rapidly with length of residence in the host country. However, the progressive development of multimorbidity among immigrants varies widely depending on area of origin. These findings provide important insight into the health care needs of specific population groups and may help minimize the negative impact of multimorbidity among the most vulnerable groups.