Objective: The virulence of COVID-19 has been particularly problematic in countries such as Spain. This led the government to decide that the population should be locked down at home to reduce the spread of the disease and avoid the collapse of the health system. Considering this, this study analyzed the changes in intimate relationships that occurred during lockdown in terms of dyadic adjustment, conflict, and quality of the relationship, as well as their relationship with anxiety symptoms. Method: Cross-sectional questionnaire-based study with adults (N = 342) aged 20-67 years who lived in Spain. Each participant completed self-report measures of anxiety (State-Trait Anxiety Inventory state and trait subscales), dyadic adjustment (Dyadic Adjustment Scale), relationship conflict and quality, and sociodemographic variables. Results: The results showed significant levels of state anxiety, which was associated with poorer dyadic adjustment and a decrease in the perceived quality of relationships since the start of lockdown. Increased partner conflict seems to be an important predictor of dyadic adjustment and relationship quality during social isolation. Conclusions: This study suggests that the COVID-19 pandemic has negatively affected the mental health of the population, especially women. This finding is closely associated with difficulties with one's cohabiting partner (e.g., worse dyadic adjustment), but the most determining factor seems to be the previous state of the relationship. Clinical Impact StatementResearch on COVID-19 and mental health has focused on exploring the state of mental health of individuals at a given point in time, without a baseline of how they were before the pandemic. In this study, we assessed anxiety as a personality trait (trait anxiety), asking participants to base their responses on how they usually felt before the onset of the pandemic (i.e., baseline). A comparison between these results and the anxiety experienced in the first wave of the pandemic (state anxiety) confirmed an increase in anxiety-related symptoms in a significant number of participants, predicting worse dyadic adjustment and a deterioration in the quality of their relationships.