Background: A high degree of burden is often experienced by informal caregivers of stroke survivors. However, little is known about the long-term impact of the caregiving role. This study sought to examine the relationship between social support and the psychological effects experienced by long-term caregivers of stroke survivors. Methods: The design was a prospective, multicentre, hospital-based stroke cohort study with 3 years of follow-up. Caregivers of participating stroke survivors completed questionnaires either through face-to-face interview or over the telephone. Primary outcome measures were the Medical Outcome Study (MOS) Social Support Survey and the Irritability, Depression, and Anxiety (IDA) Scale, and the association between them was investigated using multivariate logistic regression. Results: 174/217 (80%) stroke survivors participated at 3 years, involving 116 informal caregivers (mean age 66.9 years, 71% female). Increasing social support was correlated with lower levels of depression, anxiety and inward irritability in caregivers. Social support was independently associated with anxiety (OR 0.55, 95% CI 0.35–0.85; p = .007) and inward irritability (OR 0.47, 95% CI 0.29–0.77; p = .002). Furthermore, social support was significantly associated with carer strain, as assessed by the Caregiver Strain Index. Conclusions: The amount of social support available to long-term caregivers of stroke survivors may be an important factor in lowering psychological burden in these caregivers.