Objective: Children of mothers with severe mental illness are at significantly increased risk of developing intellectual disability. Obstetric complications are also implicated in the risk for intellectual disability. Moreover, children of mothers with severe mental illness are more likely to be exposed to obstetric complications. The purpose of this study was to examine the independent and joint contributions of familial severe mental illness and obstetric complications to the risk of intellectual disability. Method: Record linkage across Western Australian whole-population psychiatric, inpatient, birth, and midwives' registers identified 15,351 children born between 1980 and 2001 to mothers with severe mental illness and 449,229 children born to mothers with no mental illness. Multivariable models were adjusted for paternal psychiatric status, parental intellectual disability, and other family and sociodemographic covariates. Results: The risk of intellectual disability was increased among children of mothers with severe mental illness compared with children of unaffected mothers. The impact varied across maternal diagnostic groups. For children of mothers with schizophrenia, the unadjusted odds ratio was 3.8 (95% CI=3.0, 4.9) and remained significant after simultaneous adjustment for exposure to obstetric complications and other covariates (odds ratio=1.7, 95% CI=1.3, 2.3). The odds ratio for exposure to obstetric complications also remained significant after adjustment (odds ratio=1.7, 95% CI=1.6, 1.8). For intellectual disability of a genetic basis, the adjusted odds ratio for maternal schizophrenia was elevated but not statistically significant. Among children with intellectual disability, 4.2% later developed a psychotic disorder, compared with 1.1% of children without intellectual disability. Conclusions: Maternal severe mental illness and exposure to obstetric complications contribute separately to the risk of intellectual disability, suggesting potentially different causal pathways.