Background: The COVID-19 pandemic poses enormous social challenges, especially during lockdown. People with cognitive decline and their caregivers are particularly at risk of lockdown consequences. Objective: To investigate psychosocial effects in (pre-)dementia patients and caregivers during second lockdown and compare effects between first and second lockdown. Methods: We included n = 511 (pre-)dementia patients and n = 826 caregivers from the Amsterdam Dementia Cohort and via Alzheimer Nederland. All respondents completed a self-designed survey on psychosocial effects of COVID-19. We examined relations between experienced support and psychosocial and behavioral symptoms using logistic regression. In a subset of patients and caregivers we compared responses between first and second lockdown using generalized estimating equation. Results: The majority of patients (≥58%) and caregivers (≥60%) reported that family and friends, hobbies, and music helped them cope. Support from family and friends was strongly related to less negative feelings in patients (loneliness: OR = 0.3[0.1–0.6]) and caregivers (loneliness: OR = 0.2[0.1–0.3]; depression: OR = 0.4[0.2–0.5]; anxiety: OR = 0.4[0.3–0.6]; uncertainty: OR = 0.3[0.2–0.5]; fatigue: OR = 0.3[0.2–0.4]; stress: OR = 0.3[0.2–0.5]). In second lockdown, less psychosocial and behavioral symptoms were reported compared to first lockdown (patients; e.g., anxiety: 22% versus 13%, p = 0.007; apathy: 27% versus 8%, p < 0.001, caregivers; e.g., anxiety: 23% versus 16%, p = 0.033; patient’s behavioral problems: 50% versus 35%, p < 0.001). Patients experienced more support (e.g., family and friends: 52% versus 93%, p < 0.001; neighbors: 28% versus 66%, p < 0.001). Conclusion: During second lockdown, patients and caregivers adapted to challenges posed by lockdown, as psychosocial and behavioral effects decreased, while patients experienced more social support compared to first lockdown. Support from family and friends is a major protective factor for negative outcomes in patients and caregivers.