This Dutch multi-informant study examined effects of the first COVID-19 lockdown (LD; e.g., school closure and social restrictions) on parent-adolescent relationships. Four biweekly measurements before and 4 biweekly measurements during the LD were collected among adolescents (N = 179, Mage = 14.26 years, 69% girls) and their parents (N = 144, Mage = 47.01 years, 81% female). Parents' educational level was relatively diverse: 12% low (high school or lower), 33% medium (vocational training), and 55% high (college or university). Adolescents and parents reported on parental support, parent-adolescent conflict, autonomy support, psychological control, behavioral control, and time spent on various activities. Adolescents spent more time with their parents during LD (before M = 8.6 hr, during M = 12.7 hr), but less time with friends (before M = 8.1 hr, during M = 2.1 hr), and reported on average 13 COVID-19-related rules. Preregistered piecewise growth models confirmed that autonomy support decreased immediately during the LD, but no mean level changes were observed in the other relationship dimensions. During the first 2 months of the LD, parents reported gradual increases in autonomy support and decreases in behavioral control. Moreover, significant differences between families were found in sudden and more gradual relationship changes, which correlated strongly with pre-LD characteristics of the relationship, and in some models with adolescent oppositional defiance and legitimacy beliefs. In sum, findings suggest resilience in most families, but also heterogeneity: Some families were negatively affected, and others were positively affected. A tailored approach is therefore needed to mitigate the impact of COVID-19 on family functioning. (PsycInfo Database Record (c) 2021 APA, all rights reserved).