OBJECTIVE: Recent research has documented the harmful effects of ambivalence over emotional expression (AEE) on psychological well-being, but few studies to date have examined AEE among Mainland Chinese breast cancer patients, an ethnic group that prioritizes emotion restraint to preserve social harmony. The present study examined the relationship between AEE and well-being (viz, anxious and depressive symptoms and quality of life) and evaluated perceived social support as a potential mediator of this relationship in a sample of Mainland Chinese breast cancer patients. METHODS: Three hundred twenty-seven Chinese breast cancer patients recruited from Weifang, China, completed a self-reported questionnaire containing the Ambivalence over Emotional Expression Questionnaire (AEQ), the Medical Outcomes Study Social Support Scale (MOS-SSS), the Self-rating Anxiety Scale (SAS), the Self-rating Depression Scale (SDS), and the Functional Assessment of Cancer Therapy-Breast (FACT-B). RESULTS: Overall, Mainland Chinese breast cancer patients endorsed high levels of AEE. A series of mediation analyses revealed perceived social support served as a partial mediator of the relationship between AEE and well-being. Specifically, AEE was associated with lower perceived social support (βs = -.13, P < .001), which in turn, was associated with greater anxious symptoms (β = .23, P < .001), depressive symptoms (β = .20, P < .001) and lower quality of life (β = -.30, P < .001). CONCLUSIONS: The harmful relationship between AEE and well-being is partially explained by reduced social support. Psychosocial interventions that facilitate emotional disclosure without harming social harmony may be culturally effective for mainland Chinese breast cancer patients.