Objective: Given the lifelong implications of extended postpartum depression (PPD), research is needed to examine the social factors implicated in its development (such as relationship quality) and associated predictors. This study sought to examine the association of partner relationship quality (PRQ) and decline of sexual life (DSL) with maternal PPD at 12-15 months after childbirth. Methods: Prospective study of 294 low-income postpartum women. A structured questionnaire and the Patient Health Questionnaire-9 (PHQ-9) captured responses for the main outcome variable and covariates. Results: The prevalence of the main outcome (PPD at 12-15 months) was 19.1%. Using logistic regression models, low PRQ (risk ratio [RR] = 1.58, 95%CI 1.01-2.49) and DSL (RR = 1.97, 95%CI 1.23-3.15) were associated with PPD at 12-15 months even after controlling for perinatal depression. Conclusions: Late PPD (12 to 15 months after giving birth) is very common among low-income women, and is independently associated with different aspects of the couple’s relationship. Improving PRQ may prevent late PPD. Future investigations are warranted.