Background Breast cancer survivors who have completed surgery and adjuvant treatment have distinct social support needs that may relate to emotional health. There is little research on both levels of social support following treatment and the association between social support and emotional well‐being over time following breast cancer diagnosis and treatment. The aims of this study were to assess (1) the direction and magnitude of change in social support quality and quantity and (2) the degree to which change in quality and quantity of social support predicted change in emotional well‐being over time following completion of breast cancer treatment. Methods A sample of 157 female breast cancer survivors (Mage = 55, SD = 11 years) completed a baseline and a 1‐year follow‐up questionnaire assessing sociodemographic information, quality and quantity of social support, and emotional well‐being including depression symptoms, stress, and positive and negative affect. Results Social support quantity significantly decreased over 1 year, while social support quality remained stable. Based on change score analyses, a decrease in social support quality was a significant predictor of increases in depression, stress, and negative affect, explaining an extra 4 to 6% of variance in the emotional well‐being outcomes compared with social support quantity. Conclusions This study highlights the decline in social support among recently treated female breast cancer survivors and the importance of maintaining high‐quality social support for emotional well‐being. Copyright © 2016 John Wiley & Sons, Ltd.