ObjectivesTo explore the correlations among social isolation and symptoms of anxiety and depression among patients with breast cancer in China and to further verify the mediating role of social support in social isolation and symptoms of depression or anxiety.DesignA cross‐sectional survey.SettingsThe cluster sampling method was conducted for 456 female inpatients diagnosed with breast cancer at the Tumor Hospital Affiliated of Harbin Medical University from April 2019–September 2019.MethodsPearson correlation analysis was used for identifying correlations among all the variables. Mediation effect analysis was used to examine the role of social support in social isolation and symptoms of depression or anxiety.ResultsThe results showed a prevalence of 73.26% and 70.44% for anxiety and depression symptoms in patients with breast cancer, respectively. Anxiety was significantly negatively correlated with social support (r = −.334, p < .01) and significantly positively correlated with social isolation (r = .369, p < .01). Similarly, depression was significantly negatively correlated with social support (r = −.289, p < .01) and significantly positively correlated with social isolation (r = .466, p < .01). Social support played a mediating role in social isolation and f symptoms of anxiety or depression among these patients.ConclusionsSocial isolation was positively correlated with symptoms of anxiety and depression in patients with breast cancer, respectively. Social support for patients with breast cancer has a mediating effect on the patients’ social isolation and symptoms of anxiety and depression. Therefore, the support of family, friends, hospitals, and organizations plays a positive role in reducing social isolation as well as symptoms of depression and anxiety in these patients.