Abstract Background: Increased activity is beneficial during chemotherapy, but treatment-related symptoms may be a barrier. This study examines the relationship between daily fluctuations in symptoms and activity during chemotherapy. Methods: Women undergoing chemotherapy for breast cancer [n = 67; Mage = 48.6 (SD = 10.3)] wore an accelerometer 24 hours/day and received four text prompts/day to rate symptoms for 10 consecutive days at the beginning, middle, and end of chemotherapy. Mixed-effects models were used to examine the between and within-person relationships between symptom ratings on a given day and moderate to vigorous physical activity (MVPA) and light physical activity (LPA) on that day and the following day controlling for relevant covariates and using the Bonferroni correction for multiple comparisons. Results: For MVPA and LPA, within-person associations were statistically significant for same day affect, fatigue, pain, walking, activities of daily living (ADL) physical function, and cognitive function. Previous day anxiety was associated with next day LPA. Every one point worse symptom rating than an individual's overall average was associated with: (i) between 1.49 (pain) and 4.94 (fatigue) minutes less MVPA and between 4.48 (pain) and 24.72 (ADL physical function) minutes less LPA that day, and (ii) 11.28 minutes less LPA the next day. No between-person effects were significant for MVPA or LPA. Conclusions: Daily within-person variations in symptoms were associated with MVPA and LPA during chemotherapy for breast cancer. Impact: Future work should explore relationships between symptoms and activity further and identify whether tailoring to symptoms enhances efficacy of physical activity promotion interventions during chemotherapy.