We examined whether pre-disaster social support functions as a disaster preparedness resource to mitigate post-disaster depressive symptoms among older survivors of the 2011 Great East Japan earthquake and tsunami. The participants were 3,567 individuals aged ≥65 years living in Iwanuma city who completed a baseline survey as part of the nationwide Japan Gerontological Evaluation Study seven months before the disaster. A follow-up survey was administered approximately 2.5 years after the disaster. The analysis included a total of 2,293 participants, and social support (giving and receiving emotional & instrumental help) before the disaster was measured using four items. Depressive symptoms were assessed using the GDS with a cut-off score of 4/5 (not depressed/depressed). We discovered that participants who gave and received emotional and instrumental support before the disaster were significantly less likely to develop depressive symptoms after the disaster compared to those without support (ARR = 0.70; 95% CI: 0.56–0.88). The risk of the onset of depressive symptoms was 1.34 (95% CI: 1.03–1.74) among those who experienced disaster damages but had also given and received social support, and 1.70 (95% CI: 1.03–2.76) among those who experienced damages but lacked support. Strengthening social aid may help cultivate psychological resilience to disasters.