Background Frailty is conceptualized as an accumulation of deficits in multiple areas and is strongly associated with the prognosis of heart failure (HF). However, the social domain of frailty is less well investigated. We prospectively evaluated the clinical characteristics and prognostic impact of social frailty (SF) in elderly patients with HF. Methods and Results FRAGILE-HF (prevalence and prognostic value of physical and social frailty in geriatric patients hospitalized for heart failure) is a multicenter, prospective cohort study focusing on patients hospitalized for HF and aged ≥65 years. We defined SF by Makizako's 5 items, which have been validated as associated with future disability. The primary end point was a composite of all-cause death and rehospitalization because of HF. The impact of SF on all-cause mortality alone was also evaluated. Among 1240 enrolled patients, 825 (66.5%) had SF. During the 1-year observation period after discharge, the rates of the combined end point and all-cause mortality were significantly higher in patients with SF than in those without SF (Log-rank test: both P < 0.05). SF remained as significantly associated with both the combined end point (hazard ratio, 1.30; 95% CI, 1.02-1.66; P = 0.038) and all-cause mortality (hazard ratio, 1.53; 95% CI, 1.01-2.30; P = 0.044), even after adjusting for key clinical risk factors. Furthermore, SF showed significant incremental prognostic value over known risk factors for both the combined end point (net-reclassification improvement: 0.189, 95% CI, 0.063-0.316, P = 0.003) and all-cause mortality (net-reclassification improvement: 0.234, 95% CI, 0.073-0.395, P = 0.004). Conclusions Among hospitalized geriatric patients with HF, two thirds have SF. Evaluating SF provides additive prognostic information in elderly patients with HF. Registration URL: https://upload.umin.ac.jp/. Unique identifier: UMIN000023929.