Abstract Background A large number of women with breast cancer suffer from fatigue, and social support is described as having a positive impact on health in stressful life situations. The aim of this study is to evaluate social support in a sample of early-stage breast cancer outpatients with fatigue during treatment, and to evaluate the association between cancer-related fatigue and social support and between social support and demographic and treatment characteristics. Method This cross-sectional study includes 160 outpatients with early-stage breast cancer and cancer-related fatigue. The patients were recruited from clinics at a university-based cancer centre in Norway. The research instruments included The Social Provisions Scale (SPS), which measures ‛attachment’, ‛social integration’, ‛reassurance of worth’, and ‛nurturance’, and a fatigue questionnaire (FQ), which measures total, physical and mental fatigue. Data were analysed using descriptive statistics and linear regression analysis. Results Median total score for SPS was 59 (min/max = 39/64). Significant associations were found between mental fatigue and the provisions ‛reassurance of worth’ (B = − 0.34, 95% CI = [− 0.60; − 0.08]) and ‛nurturance’ (B = 0.20, 95% CI = [0.08; 0.31]). In addition, an association was found between social support and living with someone (B = 6.09, 95% CI = [4.07; 8.11]). No associations were found between physical fatigue and social support or between social support and treatment variables. Conclusions To a large extent, breast cancer patients with fatigue in this study experienced social support from their surroundings. The fact that there were significant associations between mental fatigue and two of the provisions of SPS suggests that social support is more closely related to mental fatigue than to physical fatigue. Findings from this study suggest that living with someone is important for the experience of social support during treatment for breast cancer. Clinicians need to evaluate demographic characteristics in relation to social support in early-stage breast cancer patients with fatigue.