STUDY QUESTION: Is parity associated with all-cause and cause-specific mortality among women in a nationally representative cohort of the US population, and does depression mediate this association? SUMMARY ANSWER: Nulliparous women have a higher risk of all-cause and cause-specific mortality, with depression partially mediating the relationship between parity and women's all-cause and cause-specific mortality. WHAT IS KNOWN ALREADY: Parity, a significant state in reproductive life, has enduring implications for women's health. There is also a complex relationship between depression, a prevalent mental and emotional disorder, and female fertility. Previous studies have elucidated the relationships between parity and depression, both of which are associated with mortality. However, findings from studies examining parity and women's mortality have been inconsistent. Moreover, few studies have investigated whether the effect of parity on mortality is mediated by depression. STUDY DESIGN, SIZE, DURATION: We conducted a cross-sectional study using data from seven cycles of the National Health and Nutrition Examination Survey (NHANES) spanning 2005-2018. PARTICIPANTS/MATERIALS, SETTING, METHODS: The study cohort comprised adult women with available parity and survival follow-up data. Parity data were self-reported and sourced from the Reproductive Health Questionnaire. Depression scores were derived from the Patient Health Questionnaire 9, and cause-specific deaths were identified using the International Statistical Classification of Diseases, 10th Revision (ICD-10). Weighted multivariable Cox regression was applied to analyze the association between parity, depression, and mortality. Weighted linear regression was performed to examine the relationship between parity and depression. Mediation analyses were employed to determine whether and to what extent depression mediated the effect of parity on mortality. MAIN RESULTS AND THE ROLE OF CHANCE: Our study ultimately encompassed 16 962 American women. Following multivariable adjustment, compared to nulliparous women, those with one to three live births exhibited a 17% and 33% reduction in all-cause and cancer mortality, respectively (all-cause mortality: HR = 0.83, 95% CI = 0.69-0.99, P = 0.040; cancer mortality: HR = 0.67, 95% CI = 0.45-0.99, P = 0.045). Women with more than four live births demonstrated lower all-cause mortality and mortality from other (not cancer or cardiovascular disease) diseases (all-cause mortality: HR = 0.73, 95% CI = 0.58-0.93, P = 0.011; other diseases mortality: HR = 0.66, 95% CI = 0.47-0.91, P = 0.013). No correlation was detected between parity and the risk of cardiovascular disease mortality among women. Furthermore, depression was found to partially mediate the impact of parity on all-cause mortality and mortality from other diseases in women. LIMITATIONS, REASONS FOR CAUTION: Firstly, a single index of parity was used as an exposure factor, and other reproductive factors such as birth spacing, age at first birth, and mode of delivery were not taken into account. Secondly, despite accounting for important potentially confounders in our analysis, such as BMI, smoking status, and educational level, the influence of unmeasured confounders (e.g., social class, latent reproductive system diseases) on reproductive behavior or mortality cannot be dismissed. Thirdly, women's vulnerability to depression fluctuates across reproductive stages, and the effect of depression on female fertility varies over time. Due to data constraints, we were unable to obtain information on women's mental health status at different reproductive stages. Fourthly, due to the data accessibility limitations of NHANES, we were unable to specifically explore the relationship between parity and different specific types of cancer, a limitation that may obscure potential correlations. Additionally, despite our efforts to control for various confounding factors in subgroup analyses, the smaller sample sizes in some subgroups may limit the statistical power, affecting the ability to detect effects. Finally, studies exploring the association between parity and depression are cross-sectional designs, making it difficult to infer causality. These results should be interpreted with caution, and further research is warranted to corroborate our findings. WIDER IMPLICATIONS OF THE FINDINGS: Our study underscores the elevated risk of all-cause and cause-specific mortality in nulliparous women and reveals that depression partially mediates the negative correlation between parity and women's all-cause mortality and mortality from other diseases. These results should be interpreted with caution, and further investigation is needed to support our findings. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by the National Key Research and Development Program of China (2023YFC2705700), the Key Research & Developmental Program of Hubei Province (2022BCA042), and the Interdisciplinary Innovative Talents Foundation from Renmin Hospital of Wuhan University (JCRCWL-2022-001). The authors declare that they have no conflict of interest. TRIAL REGISTRATION NUMBER: N/A.