Social isolation in older adults is associated with high rates of adverse health outcomes. Older adults who have had a recent significant health event are likely to be at risk of social isolation following hospitalization. This study aims to identify risk factors amongst older adults at hospital discharge that are associated with social isolation at three months post-hospitalization. Older adults were surveyed at hospital discharge and three months post-hospitalization. Baseline data including demographics, self-reported quality of life, physical activity and capacity levels, lifestyle factors, symptoms of depression and anxiety were collected at discharge. Social isolation was measured using the Friendship Scale at the three-month follow-up. Regression analyses were used to examine the relationship between baseline characteristics and social isolation at three months post-hospitalization. Older adults (n = 311) participated in the baseline survey, of whom 241 (78 %) completed the three-month survey. Higher depressive and anxiety symptoms at hospital discharge, comorbidity of cancer, history of cigarette smoking, prior access to community and respite service, and arrangement for shopping assistance post-discharge were factors independently associated with an increased risk of social isolation at three months post-hospitalization. This study identified risk factors for social isolation that are unique to older post-hospitalized adults. These findings can help clinicians identify individuals at risk of social isolation and to target interventions that address these risk factors for the prevention of social isolation in older adults after hospitalization.