Evidence exists that maternal depression in the perinatal period has an adverse effect on a range of early childhood outcomes and increases the risk of offspring depression during adolescence. However, the association between maternal depression during the perinatal period and offspring psychotic experiences has not been investigated. We aimed to investigate whether there is an association between maternal antenatal or postnatal depression and offspring psychotic experiences at 18 years of age.This longitudinal study used data from the Avon Longitudinal Study of Parents and Children (ALSPAC), a prospective birth cohort, which recruited 14 541 pregnant women with an estimated delivery date between April 1, 1991, and Dec 31, 1992. Perinatal depression was measured using the Edinburgh Postnatal Depression Scale (EPDS); offspring psychotic experiences at 18 years of age were measured using the Psychosis-Like Symptom Interview. Offspring of mothers with complete data on maternal perinatal depression measures, and complete data on outcome (psychotic experiences) and confounding variables were included in the main analysis. For the main analysis, we used logistic regression to examine the associations between maternal depression (antenatal and postnatal) and offspring psychotic experiences at the age of 18 years. We used biprobit regression to model the association between maternal antenatal depression and the two offspring outcomes (psychotic experiences and depression) at 18 years of age jointly.3067 offspring for whom data were available on maternal perinatal depression and offspring psychotic experiences aged 18 years were included in analyses. Maternal antenatal depressive symptoms were associated with offspring psychotic experiences at 18 years of age, with an unadjusted odds ratio (OR) of 1·38 (95% CI 1·18-1·61, p=0·0001) and after adjustment for confounders, an OR of 1·26 (1·06-1·49, p=0·0074). Maternal antenatal depressive symptoms were associated with both offspring psychotic experiences at the age of 18 years (n=2830, OR for a 5-point increase in EPDS score: 1·32 [95% CI 1·16-1·51], p<0·0001) and offspring depression at 18 years (OR for a 5-point increase in EPDS score: 1·18 [1·03-1·34], p=0·016). From joint modelling, there was no evidence that the association between maternal antenatal depression and offspring psychotic experiences differed in strength compared with offspring depression (p=0·19).The offspring of mothers who experience depression in the perinatal period are more likely to report psychotic experiences at 18 years of age. If the association is found to be causal, it would strengthen the case for identifying and treating maternal depression during and after pregnancy.UK Medical Research Council and the Wellcome Trust.