The coronavirus disease 2019 (COVID-19) pandemic and the extensive measures to prevent spread of the virus have had a major impact on the daily life of families. Such changes in family routines might have an impact on the quality of parenting and the parent-adolescent relationship. However, not all families might be affected by the crisis to a similar extent. In particular, parents and adolescents who have high levels of COVID-19-related stress might experience reduced quality of parenting or parent-child relationships. Families who are able to cope actively might experience limited or even positive changes in relationships during the COVID-19 pandemic. The current longitudinal, multi-informant, and pre-registered study used data of 240 Dutch parents and adolescents (Mage 11.4 years). Using Latent Change Score models, we examined how parent-reported parenting and adolescent-reported relationship quality changed over a six month’s period, from the pre-COVID-19 to the COVID-19 period. Moreover, we examined the moderating role of parents’ and adolescents’ COVID-19-related stress, coping, and the interaction between stress and coping on relationship change. Results showed that on average both the amount of warmth and conflict in parent-adolescent relationships decreased. For positive parenting, there was a significant interaction effect of stress and coping, with active coping leading to a mitigated decline in the quality of parenting only in families who experienced low levels of COVID-19-related stress.