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Background Social disadvantage is a key determinant of multimorbidity. Pathways through which social disadvantage leads to multimorbidity are yet undefined. In this study, we first examined the causal effect of moving into financial hardship on multimorbidity among Australian adults, and then the role of social support as a mediator of the relationship between financial hardship and multimorbidity. Methods Data were obtained from the Household, Income and Labour Dynamics in Australia (HILDA) Survey (2009–2013). We identified individuals who moved into financial hardship between 2010 and 2011 (n  = 5775). Inverse probability treatment weighting with regression adjustment was used to examine the relationship between financial hardship and multimorbidity. Causal mediation analysis was applied to decompose the total effect of financial hardship on multimorbidity into the proportion attributable to social support and the proportion not occurring through measured pathways. We accounted for baseline covariates including age, sex, marital status, educational attainment, employment status, income, country of birth, multimorbidity and social support. Bootstrapping with 1000 replications was used to calculate 95% confidence intervals (CIs). Results The risk of multimorbidity was higher in those with financial hardship by 19% [relative risk 1.19 (95% CI: 1.02–1.37) and absolute risk difference 0.036 (95% CI: 0.004–0.067)] than those without financial hardship. Social support accounted for 30% of the total effect of financial hardship on multimorbidity, risk difference 0.009 (95% CI: 0.003–0.018). Conclusions Financial hardship leads to increased risk of multimorbidity. Interventions directed at increasing social support among those in financial hardship may reduce their risk of multimorbidity. [ABSTRACT FROM AUTHOR]