Background: About one in five mothers and one in ten fathers experience postpartum depression, with adverse consequences for family health and child development. The aetiology of postpartum depressive symptoms (PPDS) is characterised within a biopsychosocial framework, yet the underlying biological mechanisms remain incompletely understood. The endocannabinoid system (ECS), which interacts with the glucocorticoid system and is implicated in the pathophysiology of depressive disorders, has not yet been investigated in relation to PPDS. Moreover, longitudinal investigations including both mothers and fathers and long-term measurements in hair are currently lacking. Objective: This study aimed to examine longitudinal associations between parents' hair endocannabinoids and parental PPDS across the first two years postpartum, adjusting for hair cortisol. Secondary aims were to assess associations between parental PPDS and hair N-acylethanolamines, and to explore intergenerational links with children's hair endocannabinoids and N-acylethanolamines. Methods: Data from 307 mothers, 208 fathers, and 288 children, who partook in the biological sub-study DREAM_HAIR were used. Mothers and fathers completed the Edinburgh Postnatal Depression Scale to assess PPDS at 8 weeks, 14 months, and 24 months after birth. At each assessment, mothers, fathers, and their children provided 2 cm hair samples. Concentrations of the endocannabinoids AEA and 1-AG/2-AG, the N-acylethanolamines SEA, PEA, and OEA, as well as cortisol were quantified. Results: Multi-group random intercept cross-lagged panel models showed no significant associations between PPDS and hair endocannabinoids or N-acylethanolamines in either mothers or fathers. Only PPDS at eight weeks after birth were associated with lower hair AEA levels at 14 months. Adjusting for hair cortisol did not alter effects. Exploratory analyses indicated that higher maternal PPDS eight weeks after birth associated with lower child hair OEA levels at 14 months, while paternal PPDS at 14 months showed a non-significant trend with higher child hair 1-AG/2-AG levels at 24 months. Conclusion: Findings suggest that rather than altered hair endocannabinoids and N-acylethanolamines predicting PPDS, depressive psychopathology may be associated with AEA signalling over time in a community cohort of mothers and fathers. Potential intergenerational effects of PPDS via the ECS require further exploration. Future research in high-risk samples and with a broader covariate adjustment is needed to inform possible interventive approaches.