The COVID-19 pandemic is placing demands on parents that may amplify the risk of parents' distress and poor parenting. Leveraging a prepandemic study in New Zealand, the current research tested whether parents' psychological distress during a mandated lockdown predicts relative residual changes in poorer parenting and whether partner support and cooperative coparenting buffer this potentially detrimental effect. Participants included 362 parents; 310 were from the same family (155 dyads). Parents had completed assessments of psychological distress and parenting prior to the pandemic and then reported on their distress, parenting, partner support, and cooperative coparenting during a nationwide COVID-19 lockdown. Parents' distress during the lockdown predicted relative residual increases in harsh parenting, but this effect was buffered by partner support. Parents' distress also predicted residual decreases in warm/responsive parenting and parent-child relationship quality, but these effects were buffered by cooperative coparenting. Partner support and cooperative coparenting are important targets for future research and interventions to help parents navigate challenging family contexts, including COVID-19 lockdowns.