Objective Physical activity (PA) has been shown to provide health benefits for breast cancer patients. The effects of augmenting oncology healthcare provider (HCP) advice for PA with 3 months of telephone counseling versus contact control were evaluated in a randomized trial. Our goal in this secondary analysis was to examine the amount of PA (min/week) needed for psychosocial benefits among both groups. Methods After receiving brief HCP advice to become physically active, 192 women (age in years: mean = 60.0, SD = 9.9) who had completed treatment for Stage 0–IV breast cancer were randomized to telephone counseling to support PA (n = 106) or contact control (n = 86). Their PA, fatigue, physical functioning, and quality of life were assessed at baseline (before receiving HCP advice), 3, 6, and 12 months. A non‐randomized design was used to examine the dose–response relationship between PA and psychosocial outcomes. Results Exercising for at least 150 min/week at moderate intensity was associated with improved physical functioning (b = 5.9, SE = 2.9, p = 0.04) and quality of life (b = 3.6, SE = 1.9, p = 0.05) at 3 months. These relationships were not found at 6 and 12 months (p's > 0.05). However, women who reported at least 150 min/week of PA at both 3 and 6 months had significantly reduced fatigue (b = 1.3, SE = 0.7, p = 0.05) and improved physical functioning (b = 3.1, SE = 1.3, p = 0.02) and quality of life (b = 2.0, SE = 0.9, p = 0.02) compared with women who did not meet this criterion. Conclusion Women who exercised at recommended levels (at least 150 min/week) and sustained this level of activity for at least 6 months accrued psychosocial benefits. Copyright © 2013 John Wiley & Sons, Ltd.