BACKGROUND: Despite being motivated to improve nutrition and physical activity behaviors, cancer survivors are still burdened by suboptimal dietary intake and low levels of physical activity. OBJECTIVE: The aim of this study was to assess changes in nutrition and physical activity behaviors after cancer diagnosis or treatment, barriers to eating a healthy diet and staying physically active, and sources for seeking nutrition advice reported by breast cancer survivors. DESIGN: This was a cross-sectional study. PARTICIPANTS/SETTING: The study included 315 survivors of breast cancer who were recruited through social media and provided completed responses to an online exploratory survey. MAIN OUTCOME MEASURES: Self-reported changes in nutrition and physical activity behaviors after cancer diagnosis or treatment, perceived barriers to healthy eating and physical activity, and sources of nutrition advice were measured. STATISTICAL ANALYSIS: Frequency distribution of nutrition and physical activity behaviors and changes, barriers to healthy eating and physical activity, and sources of nutrition advice were estimated. RESULTS: About 84.4% of the breast cancer survivors reported at least 1 positive behavior for improving nutrition and physical activity after cancer diagnosis or treatment. Fatigue was the top barrier to both making healthy food choices (72.1%) and staying physically active (65.7%), followed by stress (69.5%) and treatment-related changes in eating habits (eg, change in tastes, loss of appetite, and craving unhealthy food) (31.4% to 48.6%) as barriers to healthy eating, and pain or discomfort (53.7%) as barriers to being physically active. Internet search (74.9%) was the primary source for seeking nutrition advice. Fewer than half reported seeking nutrition advice from health care providers. CONCLUSIONS: Despite making positive changes in nutrition and physical activity behaviors after cancer diagnosis or treatment, breast cancer survivors experience treatment-related barriers to eating a healthy diet and staying physically active. Our results reinforce the need for developing tailored intervention programs and integrating nutrition into oncology care.