BACKGROUND AND AIM: Evidence suggests a positive association between cannabis use and depression however whether preconception cannabis use is associated with postpartum depression (PPD) remains unknown. This study examined the association using a longitudinal design. METHODS: Participants were from the Australian Longitudinal Study on Women's Health (cohort born in 1989-95), a sample broadly representative of similarly aged Australian women. The association of past year cannabis use at 3-15 months prior to pregnancy with PPD at ages 19-24 to 22-27 was examined in 516 women with 570 singleton live births. The associations of chronicity, recency, and initiation of cannabis use before pregnancy with PPD at ages 21-26 to 24-29 were investigated in 538 women with 590 singleton live births. PPD was ascertained from self-report of doctor diagnosis for each birth. Relative risk (RR) and 95 % confidence interval (CI) were used to calculate the associations of interest. RESULTS: Compared with no cannabis use, any past-year cannabis use at 3-15 months before pregnancy may be associated with an increased risk of PPD (1.50, 0.99-2.28). Compared with no cannabis use in the first three annual surveys before pregnancy, chronic use (a past year user in ≥ 2 surveys) was associated with an 80 % higher risk of incident PPD for births in the following 1-4 years (1.80, 1.22-2.68). Compared to never users at ages 20-25, former users who had no use within the past year had no increased risk; past year users had 73 % higher risk (1.73, 1.07-2.81); and past year users who initiated cannabis use at or after age 18 had twice the risk of incident PPD for births in the following 1-4 years (2.02, 1.17-3.49). CONCLUSION: This study provides evidence that preconception cannabis use is associated with increased risk of PPD. More studies are warranted to confirm this finding before cannabis cessation policies can be reinforced on reducing the risk of PPD.