Background: The spread of the COVID-19 virus was declared a pandemic in March 2020. New restrictions such as ‘lockdowns’ and ‘social distancing’ created challenges for individuals' work-life balance, financial situation, family life, and physical and mental health. The global population's stress levels rose in response to these changes, leading to a widespread deterioration of mental health. One group particularly affected was parents of infants and very young children. Poor parental mental health may disrupt parent-infant bonding, with negative consequences for infant well-being and development. Aims: To consider parent-infant bonding in relation to parents' perceived stress and psychological distress during the first lockdown, and to examine whether associations of stress with parent-infant bonding were mediated by parental mental health. Methods: DREAM_CORONA (n = 738: 471 mothers and 267 partners) is a sub-study of the prospective German cohort study ‘DREAM’. The SPSS modelling tool PROCESS was used to assess whether parental mental health mediated the relationship between parents' perceived stress response to the COVID-19 pandemic and parent-infant bonding, while controlling for covariates. Results: Higher levels of parental stress were strongly associated with higher levels of depression, anxiety, and aggression symptoms for both parents. In addition, there was a significant relationship between parents' self-reported stress levels and parent-infant bonding. This relationship was mediated by symptoms of depression and anxiety for fathers and by symptoms of anger-hostility for both parents. Conclusion: Increased parental stress during the early pandemic was associated with poorer parent-infant bonding. This has important implications for the management of any future public health crises.