Background: In breast cancer patients, a lack of physical activity (PA) is 1 causative factor of weight gain during adjuvant treatment. It may increase the risk of treatment adverse effects, comorbidities, and deleterious long-term outcomes. Objective: We aimed to describe the evolution of PA level and sedentary behavior in breast cancer patients between diagnosis and adjuvant chemotherapy onset following surgery and identify predictive factors associated with these changes early after breast cancer diagnosis. Methods: Baseline data of 60 patients enrolled in a pilot randomized controlled trial of PA are presented. PA levels were estimated at adjuvant chemotherapy onset after surgery and retrospectively for the period at diagnosis. Height, weight, waist circumference, and bioelectrical impedance were measured at chemotherapy onset. Linear regression analysis evaluated factors associated with relative changes of PA level and sedentary behavior. Results: Moderate PA decreased and sedentary behavior increased between diagnosis and chemotherapy onset. A grade III breast cancer was associated with a greater decrease in PA level. Conversely, keeping a professional occupation and adherence to nutritional guidelines were associated with maintained PA level. The majority (88%) of patients had excessive adiposity at chemotherapy onset. Conclusions: There was a significant deterioration of PA level between diagnosis and chemotherapy onset, and deleterious adiposity was present in most patients. Implications for practice: This study further emphasizes the need to motivate breast cancer patients toward engaging in a healthy lifestyle early after diagnosis and adhering to PA programs, which should be included in their clinical management.